drm/i915: Prepare to extract gen specific functions from intel_can_enable_sagv

Addressing one of the comments, recommending to extract platform
specific code from intel_can_enable_sagv as a preparation, before
we are going to add support for tgl+.

v2: - Removed whitespace
v3: - Removed premature debug and new cycle introduction(Ville)
    - Added missing no active pipes check(Ville)
v4: - Fixed stupid mistake with plane_state caused by stupid macro change

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/20200415145740.28241-1-stanislav.lisovskiy@intel.com