drm/vmwgfx: Fix subresource updates with new contexts

The has_dx variable was only set during the initialization which
meant that UPDATE_SUBRESOURCE was never used. We were emulating it
with UPDATE_GB_IMAGE but that's always been a stop-gap. Instead
of has_dx which has been deprecated a long time ago we need to check
for whether shader model 4.0 or newer is available to the device.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210609172307.131929-4-zackr@vmware.com