radeon: Silence GCC unused-but-set-variable warnings.

radeon: Silence GCC unused-but-set-variable warnings.

radeon_fbo.c: In function 'radeon_map_renderbuffer_s8z24':
radeon_fbo.c:162:9: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
     int ret;
         ^
radeon_fbo.c: In function 'radeon_map_renderbuffer_z16':
radeon_fbo.c:200:9: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
     int ret;
         ^
radeon_fbo.c: In function 'radeon_map_renderbuffer':
radeon_fbo.c:242:8: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
    int ret;
        ^
radeon_fbo.c: In function 'radeon_unmap_renderbuffer':
radeon_fbo.c:419:14: warning: variable 'ok' set but not used [-Wunused-but-set-variable]
    GLboolean ok;
              ^

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>