drm/i915: don't force full modeset if primary plane is disabled (v2)

In a future patch, we'll allow the primary plane to be disabled by
userspace via the universal plane API.  If a modeset is requested while
the primary plane is disabled, crtc->primary->fb will be NULL which
generally triggers a full modeset (except in fastboot situations).  If
we detect that the crtc is active, but there's no primary plane fb,
we should still allow a simple plane update rather than a full modeset
if the mode isn't actually changing (after re-enabling the primary plane
of course).

v2:
 - Enable plane after set_base to avoid enabling the plane if set_base
   fails, and to make flip+enable atomic (suggested by Ville)
 - Drop BUG to WARN if we somehow enter the 'fb_changed' modeset case
   with the crtc disabled (suggested by Ville)

Reviewed-by: Chon Ming Lee <chon.ming.lee@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

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