drm/rockchip: Test for imported buffers with drm_gem_is_imported()

drm/rockchip: Test for imported buffers with drm_gem_is_imported()

Instead of testing import_attach for imported GEM buffers, invoke
drm_gem_is_imported() to do the test. The test itself does not change.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Sandy Huang <hjc@rock-chips.com>
Cc: Heiko Stübner <heiko@sntech.de>
Cc: Andy Yan <andy.yan@rock-chips.com>
Cc: linux-rockchip@lists.infradead.org
Fixes: b57aa47d39e9 ("drm/gem: Test for imported GEM buffers with helper")
Closes: https://lore.kernel.org/dri-devel/38d09d34.4354.196379aa560.Coremail.andyshrk@163.com/
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patch.msgid.link/20260227133113.235940-11-tzimmermann@suse.de