drm/vboxvideo: drop DRM_MTRR_WC #define

Doesn't apply to upstream kernels since a rather long time.

Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200415074034.175360-3-daniel.vetter@ffwll.ch