drm/i915: Update kerneldoc for intel_dpll_mgr.c

The documentation for most of the non-static members and structs were
missing. Fix that.

v2: Fix typos (Durga)

v3: Rebase.
    Fix make docs warnings.
    Document more.

v4: capitilize CRTC; say that the prepare hook is a nop if the DPLL is
    already enabled; link to struct intel_dpll_hw_state from @hw_state
    field in struct intel_shared_dpll_state; reorganize DPLL flags; link
    intel_shared_dpll_state to other structs and functions. (Daniel)

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1483024933-3726-6-git-send-email-ander.conselvan.de.oliveira@intel.com