drm/i915: Eliminate the ugly 'fb?:' constructs from the ilk/skl wm code

drm/i915: Eliminate the ugly 'fb?:' constructs from the ilk/skl wm code

Don't access plane_state->fb until we know the plane to be visible.
It it's visible, it will have an fb, and thus we don't have to
consider the NULL fb case. Makes the code look nicer.

Cc: intel-gfx@lists.freedesktop.org
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479498793-31021-25-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>