drm/i915: No 10bit gamma on desktop gen3 parts

Apparently desktop gen3 parts don't support the
10bit gamma mode at all. Stop claiming otherwise.

As is the case with pipe A on gen3 mobile parts, the
PIPECONF gamma mode bit can be set but it has no
effect on the output.

PNV seems to be the only slight exception, but generally
the desktop PNV variant looks more like a mobile part so
this is not entirely surprising.

Fixes: 67630bac ("drm/i915: Add 10bit gamma mode for gen2/3")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230531135625.3467-1-ville.syrjala@linux.intel.com