gbm: Export a per plane getter for stride

gbm: Export a per plane getter for stride

v2: Preserve legacy behavior when plane is 0 (Jason Ekstrand)
EINVAL when input plane is greater than total planes (Jason Ekstrand)
Don't leak the image after fromPlanar (Daniel)
Move bo->image check below plane count preventing bad index succeeding (Daniel)

v3: Fix DRIimage leak (using Jason's recommended change)
Make plane 0 return planar stride. This might break legacy behavior (Jason)

v4: Move bogus hunk for get_handle_for_plane to the right patch (Jason)
Fix error handling path to be cleaner (Jason)

v5: Add assert for dumb BOs to make sure plane == 0 (Jason)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com> (v1)
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Daniel Stone <daniels@collabora.com>