drm_hwcomposer: Fix SetPowerMode() VTS tests for headless mode

Test: atest VtsHalGraphicsComposerV2_1TargetTest

Signed-off-by: Roman Stratiienko <roman.o.stratiienko@globallogic.com>
2 jobs for fix-vts-setpowermode in 9 minutes and 19 seconds (queued for 2 seconds)