Fix overwriting of config flags in a few cases

Fix overwriting of config flags in a few cases

For all of these configs, we were or'ing in the texel buffer flag, and
then setting the flags to textureable. We currently only use texel buffers
for RGBA_Float buffers, so this didn't make any difference, but I spotted
it while making other changes.

BUG=skia:

Change-Id: I76c21881b7c82c5c0561e19686ddcc62f9cca835
Reviewed-on: https://skia-review.googlesource.com/6851
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>