drm/gem: Fix sphinx warnings

Sphinx really wants colons after arguments :/

Fixes the following warnings:
drm_gem.c:1384: warning: Function parameter or member 'fence_array' not described in 'drm_gem_fence_array_add'
drm_gem.c:1384: warning: Function parameter or member 'fence' not described in 'drm_gem_fence_array_add'
drm_gem.c:1435: warning: Function parameter or member 'fence_array' not described in 'drm_gem_fence_array_add_implicit'
drm_gem.c:1435: warning: Function parameter or member 'obj' not described in 'drm_gem_fence_array_add_implicit'
drm_gem.c:1435: warning: Function parameter or member 'write' not described in 'drm_gem_fence_array_add_implicit'

Fixes: 5d5a179d ("drm: Add helpers for setting up an array of dma_fence dependencies.")
Cc: Eric Anholt <eric@anholt.net>
Cc: Qiang Yu <yuq825@gmail.com> (v1)
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190424204916.222155-1-sean@poorly.run

drivers/gpu/drm/drm_gem.c

@@ -1372,8 +1372,8 @@ EXPORT_SYMBOL(drm_gem_unlock_reservations);
  * drm_gem_fence_array_add - Adds the fence to an array of fences to be
  * waited on, deduplicating fences from the same context.
  *
- * @fence_array array of dma_fence * for the job to block on.
- * @fence the dma_fence to add to the list of dependencies.
+ * @fence_array: array of dma_fence * for the job to block on.
+ * @fence: the dma_fence to add to the list of dependencies.
  *
  * Returns:
  * 0 on success, or an error on failing to expand the array.
@@ -1423,9 +1423,9 @@ EXPORT_SYMBOL(drm_gem_fence_array_add);
  * GEM objects used in the job but before updating the reservations with your
  * own fences.
  *
- * @fence_array array of dma_fence * for the job to block on.
- * @obj the gem object to add new dependencies from.
- * @write whether the job might write the object (so we need to depend on
+ * @fence_array: array of dma_fence * for the job to block on.
+ * @obj: the gem object to add new dependencies from.
+ * @write: whether the job might write the object (so we need to depend on
  * shared fences in the reservation object).
  */
 int drm_gem_fence_array_add_implicit(struct xarray *fence_array,