ultrahdr: Avoid possible failure during gainmap compression

Allocating memory for compressed gainmap is always tricky because
the size is dependent on the data and quality factor. Too less a size
can cause intermittent errors. Avoid alloc altogether and remove
memcpy

Bug: 283503763
Test: ./libultrahdr_test

Change-Id: Ib57bac8bb25a2d00582c65067608a591f43cdc3a