drm_hwcomposer: Implement *DisplayBrightness* stubs

drm_hwcomposer: Implement *DisplayBrightness* stubs

Implement *DisplayBrightness* stub to be able boot
Android 11 using android.hardware.graphics.composer@2.3-service.
This change caused by error during boot:
failed to get hwcomposer2 function 61
61 is equal to SetDisplayBrightness.
GetDisplayBrightnessSupport is a companion of it.

Signed-off-by: Andrii Chepurnyi <andrii_chepurnyi@epam.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Change-Id: Idacb32abdcf7ac399ef989f24ab7a360248466f9