drm/i915: Do not attempt to re-enable an unconnected primary plane

Due to user fudging (for instance using video=VGA-1:e with FBDEV=n) we can
attempt to reset an inconsistent CRTC that is marked as active but has
no assigned fb. It would be wise to fix this earlier, but the long
term plan is to have primary and secondary planes associated with a
CRTC, in which crtc->fb being NULL will be expected. So for a quick
short term fix with pretensions of grandeur, just check for a NULL fb
during GPU reset and ignore the plane restoration.

This fixes a potential hard hang (a panic in the panic handler)
following a GPU hang.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
[danvet: Add a corresponding fixme comment.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>