gallium: add b5g6r5 srgb format

gallium: add b5g6r5 srgb format

GL generally doesn't seem to allow srgb formats with less (or more) than 8 bit
for the rgb channels, though some hw could easily do it (typically for formats
with up to 10 bits for the rgb channels, at least for formats with less than 8
bits support is likely widespread even). While it may be true there aren't
really any benefits for such formats, we need for it for d3d, though luckily
only for b5g6r5_srgb it seems.
So add this format along with the util code for conversion - since that util
code is heavily tuned for 8bit srgb this isn't really all that well optimized
and rounding doesn't seem right but at least it should give some halfway
meaningful results.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>