drm/omap: add omap_gem_unpin_locked()

Add omap_gem_unpin_locked() which is a version of omap_gem_unpin() that
expects the caller to hold the omap_obj lock.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191010120000.1421-3-jjhiblot@ti.com