drm/i915/bw: make struct intel_bw_state opaque

drm/i915/bw: make struct intel_bw_state opaque

With all the code touching struct intel_bw_state moved inside
intel_bw.c, we move the struct definition there too, and make the type
opaque. to_intel_bw_state() needs to be turned into a proper
function. All of this nicely reduces includes from intel_bw.h.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/743ba67e4e3c5dac4f5e58ab4d2357edea601d09.1750847509.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>