drm/mipi-dbi: 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.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Anusha Srivatsa <asrivats@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250226172457.217725-10-tzimmermann@suse.de