drm/i915: Fix new -Wint-in-bool-context gcc compiler warning

This commit fixes the following compiler warning:

drivers/gpu/drm/i915/intel_dsi.c: In function ‘intel_dsi_prepare’:
drivers/gpu/drm/i915/intel_dsi.c:1487:23: warning:
    ?: using integer constants in boolean context [-Wint-in-bool-context]
       PORT_A ? PORT_C : PORT_A),

Fixes: f4c3a88e ("drm/i915: Tighten mmio arrays for MIPI_PORT")
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170518110644.9902-1-hdegoede@redhat.com
(cherry picked from commit 0ad4dc88)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/i915_reg.h

@@ -8280,7 +8280,7 @@ enum {
 
 /* MIPI DSI registers */
 
-#define _MIPI_PORT(port, a, c)	((port) ? c : a)	/* ports A and C only */
+#define _MIPI_PORT(port, a, c)	(((port) == PORT_A) ? a : c)	/* ports A and C only */
 #define _MMIO_MIPI(port, a, c)	_MMIO(_MIPI_PORT(port, a, c))
 
 #define MIPIO_TXESC_CLK_DIV1			_MMIO(0x160004)