drm/i915/bw: make intel_atomic_get_bw_state() static

No need for this function to be accessible outside of intel_bw.c. Avoid
including the i915_drv.h mega header from other header files to make
further header cleanup easier.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6c898ec6511af47c1c5b679e516dc757cd207146.1565085691.git.jani.nikula@intel.com