drm/i915: Make intel_crtc_active() available outside intel_pm.c

Move intel_crtc_active() to intel_display.c and make it available
elsewhere as well.

intel_edp_psr_match_conditions() already has one open coded copy,
so replace that one with a call to intel_crtc_active().

v2: Copy paste a big comment from danvet's mail explaining
    when we can ditch the extra checks

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>