drm: Help unconfuse gcc, avoid accidental impossible unsigned comparisons

drivers/gpu/drm/drm_client_modeset.c: In function ‘drm_client_firmware_config’:
./include/linux/bits.h:26:28: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
   __builtin_constant_p((l) > (h)), (l) > (h), 0)))

v2: Add a warning for passing connector_count==0 as this will hit an
infinite loop, so document the invalid parameter.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200516212330.13633-2-chris@chris-wilson.co.uk

drivers/gpu/drm/drm_client_modeset.c

@@ -563,7 +563,7 @@ static bool drm_client_firmware_config(struct drm_client_dev *client,
 				       struct drm_client_offset *offsets,
 				       bool *enabled, int width, int height)
 {
-	unsigned int count = min_t(unsigned int, connector_count, BITS_PER_LONG);
+	const int count = min_t(unsigned int, connector_count, BITS_PER_LONG);
 	unsigned long conn_configured, conn_seq, mask;
 	struct drm_device *dev = client->dev;
 	int i, j;
@@ -577,6 +577,9 @@ static bool drm_client_firmware_config(struct drm_client_dev *client,
 	if (!drm_drv_uses_atomic_modeset(dev))
 		return false;
 
+	if (WARN_ON(count <= 0))
+		return false;
+
 	save_enabled = kcalloc(count, sizeof(bool), GFP_KERNEL);
 	if (!save_enabled)
 		return false;