backends/drm: don't advertise support for wide color gamut on Intel

i915 doesn't implement the Colorspace property correctly, so exposing this
feature makes it look like our HDR implementation is broken. This hides the
HDR checkbox in system settings until Intel fixes their driver

For testing purposes, users can still opt into this with the environment
variable KWIN_DRM_ALLOW_INTEL_COLORSPACE.


(cherry picked from commit 742268bf)