gbm: Clarify acceptable formats for gbm_bo

gbm: Clarify acceptable formats for gbm_bo

gbm_bo_create() was presumably meant to originally accept gbm_bo_format
enums, but it's accepted GBM_FORMAT_* tokens since the dawn of time.
This is good, since gbm_bo_format is rarely used and covers a lot less
ground than GBM_FORMAT_*.

Change the documentation to refer to both; this involves removing a 'see
also' for gbm_bo_format, since we can't also use \sa to refer to a
family of anonymous #defines.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reported-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Eric Anholt <eric@anholt.net>