drm/vc4: Test for imported buffers with drm_gem_is_imported()

drm/vc4: 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: Maxime Ripard <mripard@kernel.org>
Cc: Dave Stevenson <dave.stevenson@raspberrypi.com>
Cc: "Maíra Canal" <mcanal@igalia.com>
Cc: Raspberry Pi Kernel Maintenance <kernel-list@raspberrypi.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://patch.msgid.link/20260227133113.235940-12-tzimmermann@suse.de