drm/i915/skl: Removed assert for csr-fw-loading check during disabling dc6

drm/i915/skl: Removed assert for csr-fw-loading check during disabling dc6

As during disabling dc6 no need to check for csr firmware
loading status, so removed the assert call (Requested by Damien).

Cc: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Tested-by: Daniel Stone <daniels@collabora.com> # SKL
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446069547-24760-14-git-send-email-imre.deak@intel.com