drm/radeon/si: make sure mc ucode is loaded before checking the size

commit 8c79bae6 upstream.

Avoid a possible segfault.
Noticed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

drivers/gpu/drm/radeon/si.c

@@ -1475,11 +1475,13 @@ static int si_mc_load_microcode(struct radeon_device *rdev)
 	const __be32 *fw_data;
 	u32 running, blackout = 0;
 	u32 *io_mc_regs;
-	int i, regs_size, ucode_size = rdev->mc_fw->size / 4;
+	int i, regs_size, ucode_size;
 
 	if (!rdev->mc_fw)
 		return -EINVAL;
 
+	ucode_size = rdev->mc_fw->size / 4;
+
 	switch (rdev->family) {
 	case CHIP_TAHITI:
 		io_mc_regs = (u32 *)&tahiti_io_mc_regs;