drm/i915: Fix documentation for intel_dpll_get_freq()

Fix the following kerneldoc warning and while at it also the doc for the
corresponding vfunc hook.

$ make htmldocs 2>&1 > /dev/null | grep i915
./drivers/gpu/drm/i915/display/intel_dpll_mgr.h:285: warning: Function parameter or member 'get_freq' not described in 'intel_shared_dpll_funcs'

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200304150918.25473-1-imre.deak@intel.com