drm/amd/display: Fix null pointer issues found in emulation

[why]
fix null point issues found in emulation
Reviewed-by: Chris Park <Chris.Park@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Tested-by: Mark Broadworth <mark.broadworth@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn31/dcn31_hwseq.c

@@ -89,7 +89,8 @@ static void enable_memory_low_power(struct dc *dc)
 		REG_UPDATE(MMHUBBUB_MEM_PWR_CNTL, VGA_MEM_PWR_FORCE, 1);
 	}
 
-	if (dc->debug.enable_mem_low_power.bits.mpc)
+	if (dc->debug.enable_mem_low_power.bits.mpc &&
+		dc->res_pool->mpc->funcs->set_mpc_mem_lp_mode)
 		dc->res_pool->mpc->funcs->set_mpc_mem_lp_mode(dc->res_pool->mpc);
 
 
@@ -141,6 +142,7 @@ void dcn31_init_hw(struct dc *dc)
 
 	if (!dcb->funcs->is_accelerated_mode(dcb)) {
 		hws->funcs.bios_golden_init(dc);
+		if (hws->funcs.disable_vga)
 			hws->funcs.disable_vga(dc->hwseq);
 	}
 	// Initialize the dccg