drm/amd/display: use true, false for bool variable in display_mode_vba_21.c

drm/amd/display: use true, false for bool variable in display_mode_vba_21.c

Fixes coccicheck warning:

drivers/gpu/drm/amd/display/dc/dml/dcn21/display_mode_vba_21.c:4124:3-28: WARNING: Assignment of 0/1 to bool variable
drivers/gpu/drm/amd/display/dc/dml/dcn21/display_mode_vba_21.c:4128:5-30: WARNING: Assignment of 0/1 to bool variable
drivers/gpu/drm/amd/display/dc/dml/dcn21/display_mode_vba_21.c:5207:3-37: WARNING: Assignment of 0/1 to bool variable

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>