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After consultation with PMs, we have decided to have a bit more
brightness for SDR content (ToT: 50%, with this CL: 75%), hence
sacrificing a bit the HDR wow effect. This only applies for the
moment to kohaku (Samsung Galaxy Chromebook) and external
HDR-capable, DP-connected displays.

At the same time, using only 50%, or 75% of the brightness range
when in SDR, kills a bit the contrast ratio, which is perceived
as colors "not vivid", or greyish. This CL fixes that by adding
a bit (1.25) of gamma correction in this case in DrmDisplay.

With this CL, SDR colours look good (not washed out), and there's
no perceived brightness change in the SDR content during SDR-HDR
transitions.

Bug: 958166
Change-Id: I627514e8e31129b8b8d96e528e1e51ad8bea3d92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2317508Reviewed-by: ccameron <ccameron@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791310}