drm/i915: Sanitize the shared DPLL reserve/release interface

For consistency s/intel_get_shared_dpll()/intel_reserve_shared_dplls()/
to better match intel_release_shared_dplls(). Also, pass to the
reserve/release and get_dplls/put_dplls hooks the intel_atomic_state and
CRTC object, that way these functions can look up the old or new state
as needed.

Also release the PLLs from the atomic state via a new
put_dplls->intel_unreference_shared_dpll() call chain for better
symmetry with the reservation via the
get_dplls->intel_reference_shared_dpll() call chain.

Since nothing uses the PLL returned by intel_reserve_shared_dplls(),
make it return only a bool.

While at it also clarify the reserve/release function docbook headers
making it clear that multiple DPLLs will be reserved/released and
whether the new or old atomic CRTC state is affected.

This refactoring is also a preparation for a follow-up change that needs
to reserve multiple DPLLs.

Kudos to Ville for the idea to pass intel_atomic_state around, to make
things clearer locally where an object's old/new atomic state is
required.

No functional changes.

v2:
- Fix checkpatch issue: typo in code comment.
v3:
- Rebase on drm-tip.

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/20190628143635.22066-17-imre.deak@intel.com