drm/i915/pmu: Drop redundant IS_VALLEYVIEW check in __get_rc6()

Because VLV_GT_RENDER_RC6 == GEN6_GT_GFX_RC6, the IS_VALLEYVIEW() check is
not needed. Neither is the check present in other code paths which call
intel_rc6_residency_ns() (in functions gen6_drpc(), rc6_residency() and
rc6_residency_ms_show()).

v2: Elimintate VLV_GT_RENDER_RC6 #define (Jani)

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Badal Nilawar <badal.nilawar@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220314161310.6468-1-ashutosh.dixit@intel.com