drm/i915: Add #defines for short/long pulse on gmch platforms

For no reason at all the public docs lack them, and Dave needs them
for his hpd interrupt rework.

Cc: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --cc drivers/gpu/drm/i915/i915_reg.h
Simple merge