drm/i915: Don't dereference fb when disabling primary plane

During driver init we may not have a valid framebuffer for the primary
plane even though the plane is enabled due to failed BIOS fb takeover.
This means we have to avoid dereferencing the fb in
.update_primary_plane() when disabling the plane.

The introduction of the primary plane rotation in

 commit d91a2cb8e5104233c02bbde539bd4ee455ec12ac
 Author: Sonika Jindal <sonika.jindal@intel.com>
 Date:   Fri Aug 22 14:06:04 2014 +0530

    drm/i915: Add 180 degree primary plane rotation support

caused a regression by trying to look up the pixel format before we can
be sure there's a valid fb available. This isn't entirely unsurprising
since the rotation patches originally predate the change to the primary
plane code that calls .update_primary_plane() also when disabling the
plane:

 commit fdd508a6
 Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
 Date:   Fri Aug 8 21:51:11 2014 +0300

    drm/i915: Call .update_primary_plane in intel_{enable,
    disable}_primary_hw_plane()

v2: Warn but don't blow up when trying to enable a plane w/o an fb (Chris)

Cc: Sonika Jindal <sonika.jindal@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>