i915/i915_pm_rc6_residency: Add energy support for dgfx

igt_power library reads energy using hwmon interface for dgfx and
rapl otherwise. Modify rc6-idle and rc6-fence to use igt_power interface.

Cc: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: Riana Tauro <riana.tauro@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>