mesa: add header for share bptc decompress functions

mesa: add header for share bptc decompress functions

Move shared bptc functions to texcompress_bptc_tmp.h:
* fetch_rgba_unorm_from_block
* fetch_rgb_float_from_block
* compress_rgba_unorm
* compress_rgb_float

Create decompress functions:
* decompress_rgba_unorm
* decompress_rgb_float

Functions will be reused in gallium/auxiliary code.

v2: Add block decompress function
v3: Move all shared code to header
    Suggested-by: Marek Olšák <maraeo@gmail.com>
Signed-off-by: Denis Pauk <pauk.denis@gmail.com>
CC: Marek Olšák <maraeo@gmail.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>