main: Refactor _mesa_DrawBuffer.

main: Refactor _mesa_DrawBuffer.

This could have added a new DD table entry for DrawBuffer that takes an
arbitrary draw buffer, but, after looking at the existing DD functions,
Kenneth Graunke recommended that we just skip calling the DD functions in the
case of ARB_direct_state_access.  The DD implementations for DrawBuffer(s)
have limited functionality, especially with respect to
ARB_direct_state_access.

[Fredrik: Call the driver function when fb is the bound draw buffer]

Reviewed-by: Fredrik Höglund <fredrik@kde.org>
Signed-off-by: Fredrik Höglund <fredrik@kde.org>