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

drm/msm: 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 helper tests the dma_buf
itself while import_attach is just an artifact of the import. Prepares
to make import_attach optional.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Abhinav Kumar <quic_abhinavk@quicinc.com>
Cc: Dmitry Baryshkov <lumag@kernel.org>
Cc: Sean Paul <sean@poorly.run>
Cc: Marijn Suijten <marijn.suijten@somainline.org>
Cc: linux-arm-msm@vger.kernel.org
Cc: freedreno@lists.freedesktop.org
Acked-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250317131923.238374-7-tzimmermann@suse.de