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

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

The only caller of vmw_kms_update_implicit_fb() is the page_flip
hook which itself gets called with the plane mutex already held.
Hence we can look at plane->state safely.

v2: Drop the bogus lockdep assert

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/20180525185045.29689-4-ville.syrjala@linux.intel.com
Reviewed-by: Sinclair Yeh <syeh@vmware.com>