drm/i915: Add intel_atomic_get_bw_*_state helpers

Add correspondent helpers to be able to get old/new bandwidth
global state object.

v2: - Fixed typo in function call
v3: - Changed new functions naming to use convention proposed
      by Jani Nikula, i.e intel_bw_* in intel_bw.c file.
v4: - Change function naming back to intel_atomic* pattern,
      was decided to rename in a separate patch series.
v5: - Fix function naming to match existing practices(Ville)
v6: - Removed spurious whitespace
v7: - Removed bw_state NULL checks(Ville)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200415143911.10244-3-stanislav.lisovskiy@intel.com