Fix dimming flicker when entering layer caching.

Caching does not perform any dimming operations, which means forcing
cached layers to not dim in composer is not correct. So...dim the cached
layers as if they were SDR layers

Note that because caching does not perform any dimming, then this does
imply that some cached sets may permanently be in GPU composition if the
dimming ratio is large enough that the DPU cannot dim + dither.

If we wanted to resolve the power cost of steady-state dimming by a
large ratio, then a future patch would have to teach caching how to dim.
That approach would have a few potential difficulties:
1. Modulating the dimming ratio of cached SDR layers may cause cache
   evictions when an HDR video comes on screen, since caching will have
   to recomposite those layers due to the change in dimming ratio, which
   can be a power regression.
2. Reusing caching results that take into account dimming may be
   difficult, since dimming can cause crush at lower grey levels, so
   brightening a dimmed cached set may cause degradation in image
   quality.

(1) and (2) aren't impossible to solve, but would either require changes
to the composer interface to communicate a dimming ratio capability,
and/or would introduce complexity into caching to selecttively dim
cached results based on composer capabilities as well as what is
expected to be more efficient. Whereas this patch is pretty
straightforward.

Bug: 217794675
Test: HDR test video
Change-Id: I618a0616f49c6ae3feac5bedbb4f5b0e283f5da7