gallium/radeon: allow the winsys to choose the IB size

Picked from the amdgpu branch.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>

src/gallium/drivers/r300/r300_blit.c

@@ -382,7 +382,7 @@ static void r300_clear(struct pipe_context* pipe,
             r300_get_num_cs_end_dwords(r300);
 
         /* Reserve CS space. */
-        if (dwords > (RADEON_MAX_CMDBUF_DWORDS - r300->cs->cdw)) {
+        if (dwords > (r300->cs->max_dw - r300->cs->cdw)) {
             r300_flush(&r300->context, RADEON_FLUSH_ASYNC, NULL);
         }