drm/i915: Fix readout of PIPEGCMAX

PIPEGCMAX is a 11.6 (or 1.16 if you will) value. Ie. it can
represent a value of 1.0 when the maximum we can store in the
software LUT is 0.ffff. Clamp the value so that it gets
saturated to the max the uapi supports.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200303173313.28117-9-ville.syrjala@linux.intel.com
Reviewed-by: Swati Sharma <swati2.sharma@intel.com>