main: Refactor _mesa_ReadBuffer.

main: Refactor _mesa_ReadBuffer.

This could have added a new DD table entry for ReadBuffer that takes an
arbitrary read 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 ReadBuffer
have limited functionality, especially with respect to
ARB_direct_state_access.

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

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