drm/i915: rename sdvox_reg to hdmi_reg on HDMI context

Some (but not all) of the HDMI registers can be used to control sDVO,
so those registers have two names. IMHO, when we're talking about
HDMI, we really should call the HDMI control register "hdmi_reg"
instead of "sdvox_reg", otherwise we'll just confuse people reading
our code (we now have platforms with HDMI but without SDVO). So now
"struct intel_hdmi" has a member called "hdmi_reg" instead of
"sdvox_reg".

Also, don't worry: "struct intel_sdvo" still has a member called
"sdvo_reg".

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>