freedreno/a6xx: UBWC fixes

freedreno/a6xx: UBWC fixes

A few fixes that get UBWC working for the games/benchmarks where I
noticed problems before (in particular and manhattan, and stk (modulo
image support for UBWC when compute shaders are used for post-process
effects):

  + fix the size of the UBWC meta buffer (ie, the offset to color
    pixel data) that is returned by ->fill_ubwc_buffer_sizes()
  + correct size/layout for 8 and 16 byte per pixel formats
  + limit the supported formats.. Note all formats that can be
    tiled can be compressed.

Signed-off-by: Rob Clark <robdclark@chromium.org>