drm/i915: clarify confusion between SDVO and HDMI registers

Some HDMI registers can be used for SDVO, so saying "HDMIB" should be
the same as saying "SDVOB" for a given HW generation. This was not
true and led to confusions and even a regression.

Previously we had:
  - SDVO{B,C} defined as the Gen3+ registers
  - HDMI{B,C,D} and PCH_SDVOB defined as the PCH registers

But now:
  - SDVO{B,C} became GEN3_SDVO{B,C} on SDVO code
  - SDVO{B,C} became GEN4_HDMI{B,C} on HDMI code
  - HDMI{B,C,D} became PCH_HDMI{B,C,D}
  - PCH_SDVOB is still the same thing

v2: Rebase (v1 was sent in May 2012).

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>