PFIT_CONTROL doesn't exist on 830M, so avoid reading it in
i9xx_get_pfit_config().

Also assume that only mobile gen2/3 chipsets have a panel fitter. This
matches the documentation, but I didn't have real hardware to verify.

Gen4 docmentation is a bit inconsistent, but experimenetation on my
LPT machine suggests that the panel fitter is available on non-mobile
gen4 platforms. At least on this machine panel fitter appears works
just fine even on VGA output.
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>