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: 3b0d6458 ("drm/vmwgfx: Refuse DMA operation when SEV encryption is active")
Cc: <stable@vger.kernel.org> # v5.7+
Signed-off-by: Zack Rusin <zackr@vmware.com>
Reported-by: Nicholas Hunt <nhunt@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221201175341.491884-1-zack@kde.org

drivers/gpu/drm/vmwgfx/vmwgfx_scrn.c

@@ -949,6 +949,10 @@ int vmw_kms_sou_init_display(struct vmw_private *dev_priv)
 	struct drm_device *dev = &dev_priv->drm;
 	int i, ret;
 
+	/* Screen objects won't work if GMR's aren't available */
+	if (!dev_priv->has_gmr)
+		return -ENOSYS;
+
 	if (!(dev_priv->capabilities & SVGA_CAP_SCREEN_OBJECT_2)) {
 		return -ENOSYS;
 	}