drm/omap: add omap_gem_validate_flags()

Add a helper function omap_gem_validate_flags() which validates the
omap_bo flags passed from the userspace.

Also drop the dev_err() message, as the userspace can cause that at
will.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191010120000.1421-8-jjhiblot@ti.com