drm/gma500: Sanitize possible_clones

I doubt the DP+DP and SDVO+SDVO cloning works for this driver.
i915 at least doesn't do those. Truthfully there could be some very
specific circumstances where some of them would do doable, but
genereally it's too much pain to deal with so we've chose not to
bother. Let's use the same approach for gma500.

Also the LVDS+LVDS and DSI+DSI cases probably don't really exist as
there is one of each at most.

This does mean we'll now leave possible_clones at 0 for these encoder
types whereas previosuly we included the encoder itself in the bitmask.
But that's fine as the core now treaks 0 as a special case and adds
the encoder itself into the final bitmask reported to userspace.

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200211162208.16224-3-ville.syrjala@linux.intel.com