drm/i915: Move intel_display_power_well_is_enabled() to intel_display_power_well.c

Move intel_display_power_well_is_enabled() to intel_power_well.c, as a step
towards making the low-level power well internals (i915_power_well_ops/desc
structs) hidden.

Eventually the call to this function and in general accessing power
wells directly from elsewhere in the driver should be replaced by the
use of power domains.

No functional change.

Suggested-by: Jani Nikula <jani.nikula@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220222165137.1004194-9-imre.deak@intel.com