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 <[email protected]>
Cc: Imre Deak <[email protected]>
Cc: Sunil Kamath <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Signed-off-by: Animesh Manna <[email protected]>
[imre: removed note about reg definitions from commit message, since
 it's not relevant any more]
Signed-off-by: Imre Deak <[email protected]>
Tested-by: Daniel Stone <[email protected]> # SKL
[Jani: make assert_csr_loaded static]
Signed-off-by: Jani Nikula <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]