drm/vmwgfx: 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: Zack Rusin <zack.rusin@broadcom.com>
Cc: Broadcom internal kernel review list <bcm-kernel-feedback-list@broadcom.com>
Reviewed-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://lore.kernel.org/r/20250317131923.238374-14-tzimmermann@suse.de