drm/amd/display: avoid uninitialized variable warning

clang (quite rightly) complains fairly loudly about the newly added
mpc1_get_mpc_out_mux() function returning an uninitialized value if the
'opp_id' checks don't pass.

This may not happen in practice, but the code really shouldn't return
garbage if the sanity checks don't pass.

So just initialize 'val' to zero to avoid the issue.

Fixes: 110b055b28 ("drm/amd/display: add getter routine to retrieve mpcc mux")
Cc: Josip Pavic <Josip.Pavic@amd.com>
Cc: Bindu Ramamurthy <bindu.r@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/gpu/drm/amd/display/dc/dcn10/dcn10_mpc.c

@@ -470,7 +470,7 @@ void mpc1_cursor_lock(struct mpc *mpc, int opp_id, bool lock)
 unsigned int mpc1_get_mpc_out_mux(struct mpc *mpc, int opp_id)
 {
 	struct dcn10_mpc *mpc10 = TO_DCN10_MPC(mpc);
-	uint32_t val;
+	uint32_t val = 0;
 
 	if (opp_id < MAX_OPP && REG(MUX[opp_id]))
 		REG_GET(MUX[opp_id], MPC_OUT_MUX, &val);