drm/vmwgfx: Fix display register usage for some older configs

We can't be setting the display_id register to an invalid value
because that makes our device reset the fb which causes nasty
flicker (due to destruction and creation of a new fb).
Also we can't be using the BITS_PER_PIXEL register if the
8BIT_EMULATION is not supported.
Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Link: https://patchwork.freedesktop.org/patch/414041/?series=85516&rev=2

drivers/gpu/drm/vmwgfx/vmwgfx_kms.c

@@ -1875,7 +1875,8 @@ int vmw_kms_write_svga(struct vmw_private *vmw_priv,
 		vmw_fifo_mem_write(vmw_priv, SVGA_FIFO_PITCHLOCK, pitch);
 	vmw_write(vmw_priv, SVGA_REG_WIDTH, width);
 	vmw_write(vmw_priv, SVGA_REG_HEIGHT, height);
-	vmw_write(vmw_priv, SVGA_REG_BITS_PER_PIXEL, bpp);
+	if ((vmw_priv->capabilities & SVGA_CAP_8BIT_EMULATION) != 0)
+		vmw_write(vmw_priv, SVGA_REG_BITS_PER_PIXEL, bpp);
 
 	if (vmw_read(vmw_priv, SVGA_REG_DEPTH) != depth) {
 		DRM_ERROR("Invalid depth %u for %u bpp, host expects %u\n",

drivers/gpu/drm/vmwgfx/vmwgfx_ldu.c

@@ -125,7 +125,6 @@ static int vmw_ldu_commit_list(struct vmw_private *dev_priv)
 		vmw_write(dev_priv, SVGA_REG_DISPLAY_POSITION_Y, crtc->y);
 		vmw_write(dev_priv, SVGA_REG_DISPLAY_WIDTH, crtc->mode.hdisplay);
 		vmw_write(dev_priv, SVGA_REG_DISPLAY_HEIGHT, crtc->mode.vdisplay);
-		vmw_write(dev_priv, SVGA_REG_DISPLAY_ID, SVGA_ID_INVALID);
 
 		i++;
 	}