drm/i915/gen9: Remove state asserts when disabling DC states

Disabling the DC states when it's already disabled is a valid scenario,
for example during HW state sanitization during driver loading and
resuming or when DC states are disabled via the i915.enable_dc or
disable_power_well option.

CC: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1456778945-5411-4-git-send-email-imre.deak@intel.com