drm/i915: Tighten mmio arrays for MIPI_PORT

drivers/gpu/drm/i915/intel_dsi.c: In function ‘intel_dsi_prepare’:
drivers/gpu/drm/i915/intel_dsi.c:1308:1: error: the frame size of 2488 bytes is larger than 2048 bytes [-Werror=frame-larger-than=]

which is caused by the compiling expanding every _MIPI_PORT into an
on-stack array of u32[3] at every callsite. Not sure why only one
machine/compiler appears susceptible, but with a minor tweak to _MIPI_PORT
we can defer the error until later.

This is a partial revert of commit ce64645d ("drm/i915: use variadic
macros and arrays to choose port/pipe based registers") for a particular
bad offender.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170228145519.18012-1-chris@chris-wilson.co.ukAcked-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/i915_reg.h

@@ -8248,7 +8248,7 @@ enum {
 
 /* MIPI DSI registers */
 
-#define _MIPI_PORT(port, a, c)	_PORT3(port, a, 0, c)	/* ports A and C only */
+#define _MIPI_PORT(port, a, c)	((port) ? c : a)	/* ports A and C only */
 #define _MMIO_MIPI(port, a, c)	_MMIO(_MIPI_PORT(port, a, c))
 
 #define MIPIO_TXESC_CLK_DIV1			_MMIO(0x160004)