drm/vmwgfx: Add an option to change assumed FB bpp

Offer an option for advanced users who want larger modes at 16bpp.

This becomes necessary after the fix: "Work around mode set
failure in 2D VMs."  Without this patch, there would be no way
for existing advanced users to get to a high res mode, and the
regression is they will likely get a black screen after a software
update on their current VM.

Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Cc: <stable@vger.kernel.org>
diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_kms.c b/drivers/gpu/drm/vmwgfx/vmwgfx_kms.c
index 077f16d..e29da45 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_kms.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_kms.c
@@ -1555,6 +1555,9 @@
 	int i;
 	u32 assumed_bpp = 4;
 
+	if (dev_priv->assume_16bpp)
+		assumed_bpp = 2;
+
 	if (dev_priv->active_display_unit == vmw_du_screen_target) {
 		max_width  = min(max_width,  dev_priv->stdu_max_width);
 		max_height = min(max_height, dev_priv->stdu_max_height);