drm/i915: Clean up fbc vs. plane checks

drm/i915: Clean up fbc vs. plane checks

Let's record the information whether a plane can do fbc or not under
struct inte_plane.

v2: Rebase due to i9xx_plane_id
    Handle BDW/HSW correctly
v3: Move inte_fbc_init() back since we depend on it happening
    even with i915.disable_display, and populate
    fbc->possible_framebuffer_bits directly from the
    plane init code instead
v4: Add note about plane A being tied to pipe A on HSW+

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180221173101.19385-1-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180221160235.11134-5-ville.syrjala@linux.intel.com