drm/i915: Extract physical display dimensions from VBT

The VBT has these mysterious H/V image sizes as part of the display
timings. Looking at some dumps those appear to be the physical
dimensions in mm. Which makes sense since the timing descriptor matches
the format used by EDID detailed timing descriptor, which defines these
as "H/V Addressable Video Image Size in mm".

So let's use that information from the panel fixed mode to get the
physical dimensions for LVDS/eDP/DSI displays. And with that we can
fill out the display_info so that userspace can get at it via
GetConnector.

v2: Use (hi<<8)|lo instead of broken (hi<<4)+lo
    Handle LVDS and eDP too

Cc: Stephen Just <stephenjust@gmail.com>
Tested-by: Stephen Just <stephenjust@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96255


Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464685714-30507-1-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Mika Kahola <mika.kahola@intel.com>
(cherry picked from commit df457245)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>