drm/i915/display: remove explicit CNL handling from intel_display_power.c

The only real platform with DISPLAY_VER == 10 is GLK. We don't need to
handle CNL explicitly in intel_display_power.c.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210728215946.1573015-14-lucas.demarchi@intel.com