Only scale SDR luminance by content max luminance for tonemapping

If color conversion is SDR->SDR, then scale luminance only by the
display luminance.

The use-case is for applying a color transform, which was defined to
occcur in linear space. Since uniform generation for tonemapping is (for
now) dataspace-agnostic, this means that scaling by the content
luminance should not be used since a tone-mapping operator may override
the one that is provided by applications.

Bug: 200310159
Test: builds, boots
Test: librenderengine_test
Change-Id: I2cc5860b6df064664c5e462674df20b589eae291