drm/i915: Use _MMIO_PIPE2() where appropriate

Stop hand rolling _MMIO_TRANS2() and just use the real thing.
Note that this register isn't even used atm, hence why this
builds despite the _PIPE2() macro not actually existing.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220623130900.26078-8-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/i915_reg.h

@@ -4383,7 +4383,7 @@
 #define DSPLINOFF(plane)	DSPADDR(plane)
 #define DSPOFFSET(plane)	_MMIO_PIPE2(plane, _DSPAOFFSET)
 #define DSPSURFLIVE(plane)	_MMIO_PIPE2(plane, _DSPASURFLIVE)
-#define DSPGAMC(plane, i)	_MMIO(_PIPE2(plane, _DSPAGAMC) + (5 - (i)) * 4) /* plane C only, 6 x u0.8 */
+#define DSPGAMC(plane, i)	_MMIO_PIPE2(plane, _DSPAGAMC + (5 - (i)) * 4) /* plane C only, 6 x u0.8 */
 
 /* CHV pipe B blender and primary plane */
 #define _CHV_BLEND_A		0x60a00