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.

Maybe we should also remove the HDR capability on eDP with Intel, as it's entirely broken / not actually fully implemented for Intel laptop panels (https://gitlab.freedesktop.org/drm/intel/-/issues/8425), but removing the wcg capability already makes the "HDR" checkbox be not exposed in the GUI, which is the most important part.