mesa: enable msaa in clear_with_quad if needed

If the DrawBuffer sample count is > 1 and msaa is enabled we must also
enable msaa when clearing it.

Fixes: ea5b7de1 ("radeonsi: make gl_SampleMaskIn = 0x1 when MSAA is disabled")
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1991



Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Witold Baryluk <witold.baryluk@gmail.com>
(cherry picked from commit 8a723282)