drm/i915/dpll: drop unused but set variables bestn and bestm1

Prepare for re-enabling -Wunused-but-set-variable.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a85ccc8c0f451fcb997b4ac138dbeba2a653cebe.1685119007.git.jani.nikula@intel.com