drm/i915: fix LFP data fetch

Apparently the proper way to do this is to use the LFP data pointer
block to figure out the LFP data block entry size, then use that plus
the panel index to calculate an offset into the LFP data block array.

Similar fix has already been pushed to the 2D driver to fix fdo bug
applied to the VBIOS reader, and things look sane).

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>