drm/gem: Fix a GEM leak in drm_gem_get_unmapped_area()

drm/gem: Fix a GEM leak in drm_gem_get_unmapped_area()

drm_gem_object_lookup_at_offset() can return a valid object with
filp or filp->f_op->get_unmapped_area set to NULL. Make sure we still
release the ref we acquired on such objects.

Cc: Loïc Molinari <loic.molinari@collabora.com>
Fixes: 99bda20d6d4c ("drm/gem: Introduce drm_gem_get_unmapped_area() fop")
Reviewed-by: Loïc Molinari <loic.molinari@collabora.com>
Link: https://patch.msgid.link/20260106164935.409765-1-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>