drm/omap: gem: Don't take struct_mutex to get GEM object mmap offset

GEM objects mmap offsets are created by calling
drm_gem_create_mmap_offset_size() that doesn't need struct_mutex
protection as it includes its own locking, based on a size that is
static across the object's life time. Remove the unneeded struct_mutex
locking.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>