drm/i915/gen9: Fix DMC firmware initialization

In commit 1e657ad7 we moved the last step of firmware initialization to
skl_display_core_init(), where it will be run only during system resume,
but not during driver loading. Since this init step needs to be done
whenever we program the firmware fix this by moving the initialization
to the end of intel_csr_load_program().

While at it simplify a bit csr_load_work_fn().

This issue prevented DC5/6 transitions, this change will re-enable those.

v2:
- remove debugging left-over and redundant comment in csr_load_work_fn()

Fixes: 1e657ad7

 ("drm/i915/gen9: Write dc state debugmask bits only once")
CC: Mika Kuoppala <mika.kuoppala@intel.com>
CC: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1457121461-16729-1-git-send-email-imre.deak@intel.com