drm/vmwgfx: Use dma_buf from GEM object instance

Avoid dereferencing struct drm_gem_object.import_attach for the
imported dma-buf. The dma_buf field in the GEM object instance refers
to the same buffer. 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/20250317131923.238374-15-tzimmermann@suse.de