drm/i915: rename I915_PMU_MAX_GTS to I915_PMU_MAX_GT

_GTS as an abbreviation here leads to some confusion, match other
definitions and drop the s.

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>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230602231754.1596433-3-matthew.s.atwood@intel.com