drm/i915: fixup 12bpc hdmi dotclock handling

We need to multiply the hdmi port dotclock by 1.5x since it's not
really a dotclock, but the 10/8 encoding bitclock divided by 10.

Also add correct limit checks for the dotclock and reject modes which
don't fit. HDMI 1.4 would allow more, but our hw doesn't support that
unfortunately :(

Somehow I suspect 12bpc hdmi output never really worked - we really
need an i-g-t testcase to check all the different pixel modes and
outputs.

v2: Fixup the adjusted port clock handling - we need to make sure that
the fdi link code still gets the real pixelclock.

v3: g4x/vlv don't support 12bpc hdmi output so drop the bogus comment.

Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Switch dotclock limit check to <= as suggested by Ville.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>