drm/i915: Refactor bdw_get_pipemisc_bpp

Move bdw_get_pipemisc_bpp alongside bdw_set_pipemisc

Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1556809862-31203-2-git-send-email-vandita.kulkarni@intel.com