drm: Add some missing forward declarations

The drm_file and drm_clip_rect structures are used throughout the file
but they are never declared nor pulled in through an include. Add
forward declarations to make them available.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>