drm/i915: Extract skl SAGV checking

Introduce platform dependent SAGV checking in
combination with bandwidth state pipe SAGV mask.

This is preparation to adding TGL support, which
requires different way of SAGV checking.

v2, v3, v4, v5, v6: Fix rebase conflict

v7: - Nuke icl specific function, use skl
      for icl as well, gen specific active_pipes
      check to be added in the next patch(Ville)

v8: - Use more generic intel_crtc_can_enable_sagv
      for checking(Ville)

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/20200513093816.11466-3-stanislav.lisovskiy@intel.com