mesa: fix error handling in DrawBuffers

mesa: fix error handling in DrawBuffers

Patch rearranges error checking so that enum checking provided via
destmask happens before other checks. It needs to be done in this
order because other error checks do not work properly if there were
invalid enums passed.

Patch also refines one existing check and it's documentation to match
GLES 3.0 spec (also in later specs). This was somewhat mysteriously
referring to desktop GL but had a check for gles3.

Fixes following dEQP tests:

   dEQP-GLES31.functional.debug.negative_coverage.get_error.buffer.draw_buffers

no CI regressions observed.

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98134
Cc: "12.0 13.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>