drm/i915/pm: drop get_fifo_size vfunc.

The i845_update_wm code was always calling the i845 variant,
and the i9xx_update_wm had only a choice between i830 and i9xx
paths, hardly worth the vfunc overhead.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/07523b1e46cd10adb2991ed4d2619b542a48c1ce.1632869550.git.jani.nikula@intel.com