drm/vmwgfx: Also check for SVGA_CAP_DX before reading DX context support

Virtual device consider SVGA_CAP_DX and SVGA3D_DEVCAP_DXCONTEXT
independent of each other. Some of the commands in cmd_buf depends on
SVGA_CAP_DX, so better to check for that as well.

Signed-off-by: Deepak Rawat <drawat.floss@gmail.com>
Reviewed-by: Thomas Hellström (VMware) <thomas_os@shipmail.org>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Signed-off-by: Roland Scheidegger <sroland@vmware.com>