drm/i915: Nuke _MMIO_PLANE_GAMC()

_MMIO_PLANE_GAMC() is some leftover macro that is never used.
Get rid of it.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240510152329.24098-2-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/i915_reg.h

@@ -5151,8 +5151,6 @@ enum skl_power_gate {
 #define  WM_DBG_DISALLOW_MAXFIFO	(1 << 1)
 #define  WM_DBG_DISALLOW_SPRITE		(1 << 2)
 
-#define _MMIO_PLANE_GAMC(plane, i, a, b)  _MMIO(_PIPE(plane, a, b) + (i) * 4)
-
 /* Plane CSC Registers */
 #define _PLANE_CSC_RY_GY_1_A	0x70210
 #define _PLANE_CSC_RY_GY_2_A	0x70310