freedreno/a6xx: use single format enum

freedreno/a6xx: use single format enum

Loses some information about which formats can be used in which cases, but
we encode that information in the format table anyway.

Important notes:
* RB6_R10G10B10A2_UNORM becomes FMT6_R10G10B10A2_UNORM_DEST
* TFMT6_8_8_8_UNORM becomes FMT6_8_8_8_X8_UNORM (not FMT6_8_8_8_UNORM)

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3798>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3798>