disable c6 called in guc_pc_fini_hw is unreachable.

GuC PC init returns earlier if skip_guc_pc is true and never
registers the finish call thus making disable_c6 unreachable.

move this call to gt idle.

v2: rebase
v3: add fixes tag (Himal)

Fixes: 975e4a37 ("drm/xe: Manually setup C6 when skip_guc_pc is set")
Signed-off-by: Riana Tauro <riana.tauro@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240606100842.956072-3-riana.tauro@intel.comSigned-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>