MAINTAINERS: Add maintainer entry for the VMWGFX DRM driver

Thomas is one of the original authors of the driver, with recent
contributions from Sinclair and Brian.

v2: Add Sinclair as maintainer. Add Sinclair+Thomas's tree, use
Supported as status.

Cc: Sinclair Yeh <syeh@vmware.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: Brian Paul <brianp@vmware.com>
Cc: "VMware Graphics" <linux-graphics-maintainer@vmware.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1462230939-26389-1-git-send-email-emil.l.velikov@gmail.com