drm/i915: Remove spll_refcount for hsw

SPLL would be a reference clock we could potentially share,
especially if we want to use the SSC mode. But currently we
don't, so let's rip out this complexity for a simpler conversion
to the new display pll framework.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>