drm/vmwgfx: Fix prime with external buffers

drm/vmwgfx: Fix prime with external buffers

Make sure that for external buffers mapping goes through the dma_buf
interface instead of trying to access pages directly.

External buffers might not provide direct access to readable/writable
pages so to make sure the bo's created from external dma_bufs can be
read dma_buf interface has to be used.

Fixes crashes in IGT's kms_prime with vgem. Regular desktop usage won't
trigger this due to the fact that virtual machines will not have
multiple GPUs but it enables better test coverage in IGT.

Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Fixes: b32233acceff ("drm/vmwgfx: Fix prime import/export")
Cc: <stable@vger.kernel.org> # v6.6+
Cc: Broadcom internal kernel review list <bcm-kernel-feedback-list@broadcom.com>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v6.9+
Link: https://patchwork.freedesktop.org/patch/msgid/20240816183332.31961-3-zack.rusin@broadcom.com
Reviewed-by: Martin Krastev <martin.krastev@broadcom.com>
Reviewed-by: Maaz Mombasawala <maaz.mombasawala@broadcom.com>