drm/i915/display: Refactor DPKGC code to call it from atomic_commit_tail

drm/i915/display: Refactor DPKGC code to call it from atomic_commit_tail

Refactor the code to check the fixed refresh rate condition in the dpkgc
function itself and call it from intel_atomic_commit_tail so that we
have all the required values specially linetime which is computed after
intel_wm_compute, this will also help implement some WA's which requires
linetime. This also avoid writing into any of the registers while we are
in compute_config phase.

Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241203084706.2126189-4-suraj.kandpal@intel.com