drm/amd/display: fix boolconv.cocci warning

./drivers/gpu/drm/amd/display/dc/dcn32/dcn32_mpc.c:729:63-68:
WARNING: conversion to bool not needed here

Generated by: scripts/coccinelle/misc/boolconv.cocci
Signed-off-by: Yihao Han <hanyihao@vivo.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>