Un-inline drm_legacy_findmap() to not depend on struct drm_device
definition within drm_legacy.h, so that a forward declaration suffices.

Also include drm_hashtab.h in drm_legacy.h to make it more
self-contained. Make it easier to drop drmP.h includes.

v2: avoid including drm_device.h by un-inlining (Daniel)

[Updated commit message per Laurent's review while applying.]

Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181228130446.22141-1-jani.nikula@intel.com