drm/i915/dp: Fix buffer size for sink_irq_esi read

The buffer size defined is 16 bytes whereas only 14 bytes are read. Add a
macro to avoid this discrepancy.

Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: James Ausmus <james.ausmus@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170918222141.4674-3-dhinakaran.pandiyan@intel.com