drm_hwcomposer: Modify source_layers_ to be integer instead of array

It isn't used as an array. Simplify the logic.

Signed-off-by: Roman Stratiienko <roman.o.stratiienko@globallogic.com>
Tested-by: Martin Juecker <martin.juecker@gmail.com>
Reviewed-by: Matvii Zorin <matvii.zorin@globallogic.com>
2 jobs for cleanup-compositor-2021w50 in 9 minutes and 35 seconds (queued for 1 second)