backends/drm: Make sure we respect the alpha property range

This is in part a workaround for an issue in Qualcomm drivers where the range for alpha is 0-UINT8_MAX (255). We were setting a value too big and this would make the whole pipeline fail.

Instead set the top of the property range which is what we were doing after all, resulting in more explicit code to what we were trying to achieve.

Additionally, include a warning if we are ever setting an invalid value, this can save some of us quite some time in the future, since failing to fall in the range makes the whole pipeline become rejected.

Signed-off-by: Victoria Fischer victoria.fischer@mbition.io