drm/i915: Draw a picture about video timings

The docs are a bit lacking when it comes to describing when certain
timing related events occur in the hardware. Draw a picture which
tries to capture the most important ones.

v2: Clarify a few details (Imre)
v3: Add HSW+ HDMI scanline counter numbers

Acked-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Akash Goel <akash.goels@gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: "Sourab Gupta <sourabgupta@gmail.com&gt;">
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>