drm_hwcomposer: Avoid filling the log with warnings when test compositions fail

We frequently test compositions that might fail due to various
reasons. On db820 we see:

 W HwcComposer: command 0x4010000 generated error 8
 E hwc-drm-display-compositor: Alpha is not supported on plane 29
 E hwc-drm-two: Failed to apply the frame composition ret=-22

over and over

So to avoid filling the log with errors when those test
compsitions don't work out, this patch tries to quiet down those
error messages when we are doing test compositions.

Change-Id: Iab19628bd7488c7d104e2c775310989bb59e140d
Signed-off-by: John Stultz <john.stultz@linaro.org>
1 job for avoid-alpha-warnings in 39 seconds (queued for 3 seconds)