drm_hwcomposer: Rework ValidateDisplay layer check to use if statement rather then switch

AOSP's toolchain throws errors on un-annotated switch case
fallthroughs. Rather then adding [[fallthrough]] annotations,
which would add C++17 syntax, switch to using a if statement
instead.

Change-Id: Id0b2bf6d365d50e637569f0c4353ceb4fda21c16
Signed-off-by: John Stultz <john.stultz@linaro.org>
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v2: Rework conditional to be more readable as suggested by seanpaul