drm/i915/gen9: extract parse_csr_fw

The loader function will get a bit more complicated soon, extract the
parsing code to make the control flow clearer. While doing that just
use dev_priv->csr.dmc_payload as the indicator for whether it all
suceeded or not.

v2-v3:
- unchanged
v4:
- rebased on top of latest drm-intel-nightly

Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Sunil Kamath <sunil.kamath@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
[imre: remove note about BE cast from commit message, it's not relevant
 any more]
Signed-off-by: Imre Deak <imre.deak@intel.com>
Tested-by: Daniel Stone <daniels@collabora.com> # SKL
[Jani: fix checkpatch warn on multiple blank lines]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447341089-2735-1-git-send-email-imre.deak@intel.com