drm/vmwgfx: Enable DMA mappings with SEV

Enable DMA mappings in vmwgfx after TTM has been fixed in commit
3bf3710e ("drm/ttm: Add a generic TTM memcpy move for page-based iomem")

This enables full guest-backed memory support and in particular allows
usage of screen targets as the presentation mechanism.

Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Reported-by: Ye Li <ye.li@broadcom.com>
Tested-by: Ye Li <ye.li@broadcom.com>
Fixes: 3b0d6458 ("drm/vmwgfx: Refuse DMA operation when SEV encryption is active")
Cc: Broadcom internal kernel review list <bcm-kernel-feedback-list@broadcom.com>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v6.6+
Reviewed-by: Martin Krastev <martin.krastev@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240408022802.358641-1-zack.rusin@broadcom.com