MAINTAINERS: update vmwgfx info

Roland will be focusing on lavapipe over the next few months and
won't have time for vmwgfx.
vmwgfx is now maintained within drm-misc.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210615182336.995192-1-zackr@vmware.com