drm/i915/gen9: move assert_csr_loaded into intel_rpm.c

drm/i915/gen9: move assert_csr_loaded into intel_rpm.c

Avoids non-static functions since all the callers are in intel_rpm.c.

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: removed note about reg definitions from commit message, since
 it's not relevant any more]
Signed-off-by: Imre Deak <imre.deak@intel.com>
Tested-by: Daniel Stone <daniels@collabora.com> # SKL
[Jani: make assert_csr_loaded static]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446069547-24760-4-git-send-email-imre.deak@intel.com