drm/vmwgfx: Stop using plane->fb in atomic_enable()

drm/vmwgfx: Stop using plane->fb in atomic_enable()

Instead of looking at the (soon to be deprecated) plane->fb we'll
examing plane->state->fb instead. We can do this because
vmw_du_crtc_atomic_check() prevents us from enabling a crtc
without the primary plane also being enabled.

Due to that same reason, I'm actually not sure what the checks here are
for NULL fb. If we can't enable the crtc without an enabled plane
we should always have an fb. But I'll leave that for someone else
to figure out.

Cc: Deepak Rawat <drawat@vmware.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: Sinclair Yeh <syeh@vmware.com>
Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180525185045.29689-6-ville.syrjala@linux.intel.com
Reviewed-by: Sinclair Yeh <syeh@vmware.com>