mesa: Restyle _mesa_check_blend_func_error().

mesa: Restyle _mesa_check_blend_func_error().

I'm about to add more error conditions to this function, so I wanted to
move the current spec citation above the code that checks it.  Indenting
it required reformatting, so I tried to move it to our newer style.

While there, I also decided to drop some GL type usage, and drop the
unnecessary "_mesa_" prefix on a static function.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>