drm/i915: Fix g4x+ sprite dotclock limit for upscaling

drm/i915: Fix g4x+ sprite dotclock limit for upscaling

Even if we're not doing downscaling we should account for
some of the extra dotclock limitations for g4x+ sprites. In
particular we must never exceed the 90% rule, and with RGB
that limits actually drops to 80%.

So instead of bailing out when upscaling let's clamp the
scaling factor appropriately and go through the rest of
calculation normally. By luck we already did the full
calculations for the 1:1 case.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200206201204.31704-1-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>