drm/i915: Reduce I915_MAX_GT to 2

According to Ashutosh there is no current or planned product in i915 for
I915_MAX_GT to be 4 anymore.

Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Ashutosh Dixit <ashutosh.dixit@linux.intel.com>
Cc: Andi Shyti <andy.shyti@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>

Signed-off-by: Matt Atwood <matthew.s.atwood@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230602231754.1596433-2-matthew.s.atwood@intel.com