drm/nouveau/kms/nv50-gp1xx: add WAR for EVO push buffer HW bug

Thanks to NVIDIA for confirming this workaround, and clarifying which HW
is affected.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Tested-by: Alexander Kapshuk <alexander.kapshuk@gmail.com>

drivers/gpu/drm/nouveau/dispnv50/disp.c

@@ -257,6 +257,12 @@ nv50_dmac_create(struct nvif_device *device, struct nvif_object *disp,
 	dmac->push->end = dmac->push->bgn;
 	dmac->max = 0x1000/4 - 1;
 
+	/* EVO channels are affected by a HW bug where the last 12 DWORDs
+	 * of the push buffer aren't able to be used safely.
+	 */
+	if (disp->oclass < GV100_DISP)
+		dmac->max -= 12;
+
 	args->pushbuf = nvif_handle(&dmac->_push.mem.object);
 
 	ret = nv50_chan_create(device, disp, oclass, head, data, size,