drm:dcn31: fix boolreturn.cocci warnings

./drivers/gpu/drm/amd/display/dc/dcn31/dcn31_panel_cntl.c:112:9-10:WARNING:
return of 0/1 in function 'dcn31_is_panel_backlight_on'
with return type bool

./drivers/gpu/drm/amd/display/dc/dcn31/dcn31_panel_cntl.c:122:9-10:WARNING:
return of 0/1 in function 'dcn31_is_panel_powered_on'
with return type bool

Return statements in functions returning bool should use true/false
instead of 1/0.

Generated by: scripts/coccinelle/misc/boolreturn.cocci

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Jing Yangyang <jing.yangyang@zte.com.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>