Using compound literals for initialization can be tricky. Lacking a
const qualifier, they won't end up in rodata, which is probably not
expected or intended. Add const to move a whopping 136 initializers to
rodata.

Compare:

$ objdump --syms drivers/gpu/drm/i915/display/intel_display_power_map.o | grep "\.rodata.*__compound_literal"
$ objdump --syms drivers/gpu/drm/i915/display/intel_display_power_map.o | grep "\.data.*__compound_literal"

Before and after the change.

Fixes: c32ffce4 ("drm/i915: Convert the power well descriptor domain mask to an array of domains")
Fixes: 4a845ff0 ("drm/i915: Simplify power well definitions by adding power well instances")
Cc: Imre Deak <imre.deak@intel.com>
Cc: Jouni Högander <jouni.hogander@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220429142140.2671828-1-jani.nikula@intel.com