drm/i915: Fix wrong number of HDMI translation entries

I keep telling myself that those tables aren't great because their size
is the number of dwords we need to program and not the number of entries
(number of dwords = number of entries * 2).

And... I got it wrong when I refactored the code. Fortunately, it was
only wrong when the VBT table (or the code parsing it) is itself
erroneous. Long story short, it shouldn't matter, but still, there's a
potential array overflow and random programming of the DDI translation
tables.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>