drm/vmwgfx: Don't use screen objects when SEV is active

drm/vmwgfx: Don't use screen objects when SEV is active

When SEV is enabled gmr's and mob's are explicitly disabled because
the encrypted system memory can not be used by the hypervisor.

The driver was disabling GMR's but the presentation code, which depends
on GMR's, wasn't honoring it which lead to black screen on hosts
with SEV enabled.

Make sure screen objects presentation is not used when guest memory
regions have been disabled to fix presentation on SEV enabled hosts.

Fixes: 3b0d6458c705 ("drm/vmwgfx: Refuse DMA operation when SEV encryption is active")
Cc: <[email protected]> # v5.7+
Signed-off-by: Zack Rusin <[email protected]>
Reported-by: Nicholas Hunt <[email protected]>
Reviewed-by: Martin Krastev <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]