drm/i915: remove IS_LP()

drm/i915: remove IS_LP()

The .is_lp member of struct intel_device_info and its wrapper IS_LP()
are used to identify just four platforms, VLV/CHV/BXT/GLK. It didn't
become as important as it was perhaps originally planned. Just remove
it, and replace with exact platform identification. In a few places this
becomes slightly verbose, but in many places it improves clarity to
immediately see the exact platforms.

Additionally, this lets us remove the xe compat macro.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240930124056.3541988-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>