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