ultrahdr: revert gain map image dimension calculation

Gain map resolution is 1 / kMapDimensionScaleFactor of primary image.
Before computing the gain map image res, the primary image res is
aligned to kMapDimensionScaleFactor and divided. This would require
p010 and 420 to have additional bytes at each row end during access or
clamp the reads to width. Currently neither is present. Until this is
handled, adjust gain map res computations to avoid oob reads.

Bug: 294709120
Bug: 294320724
Test: ./ultrahdr_enc_fuzzer

Change-Id: I65daa5ac49255a8976cf929a5bd0ffe3e47afb6d