backends/drm: handle missing brightness values in the EDID better
Most importantly, fall back to an assumption of 1000 nits peak brightness when the max brightness is missing, instead of the sdr brightness, which causes more immediately visible issues. Ideally the user should configure this value in the display settings, but it's too late to still do that for Plasma 6.0.
The second commit just cleans up the code around this a bit, because I saw there was lots of unnecessary duplication in DrmPipeline + DrmOutput