drm/i915: Sanitize the shared DPLL find/reference interface

Pass the PLL HW state to the PLL find/reference functions making it
clearer what is their input. Also pass to these the atomic state and the
CRTC object instead of the CRTC state, since they don't require the
latter.

Move setting the PLL in the crtc_state to the get_dpll() hook, which
is the more logical place for this, where the related PLL HW state was also
set.

This refactoring is also a preparation for a follow-up patch that will
have to find/reference multiple PLLs.

No functional changes.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190628143635.22066-18-imre.deak@intel.com