Pass a crtc_state to it and find whether the pipe has an encoder of a
given type by looking at the drm_atomic_state the crtc_state points to.

Until recently i9xx_get_refclk() used to be called indirectly from
vlv_force_pll_on() with a dummy crtc_state. That dummy crtc state is not
converted to be part of a full drm atomic state, so add a WARN in case
someone decides to call that again with a such dummy state. This was
removed in

commit 9cbe40c1
Author: Vijay Purushothaman <vijay.a.purushothaman@linux.intel.com>
Date:   Thu Mar 5 19:33:08 2015 +0530

    drm/i915: Update prop, int co-eff and gain threshold for CHV

v2: Warn if there is no connectors for a given crtc. (Daniel)
    Replace comment i9xx_get_refclk() with a WARN_ON(). (Ander)
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
[danvet: Add commit reference for when i9xx_get_refclk was removed.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>