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