drm/i915/gt: Use consistent offset notation in intel_gt_regs.h

Switch all register offsets to use lowercase hex values for consistency.
Also strip any unnecessary leading 0's.  For example, "_MMIO(0x0D08)"
becomes "_MMIO(0xd08)."

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Atwood <matthew.s.atwood@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220209051140.1599643-6-matthew.d.roper@intel.com