drm/i915: Do not call hsw_set_frame_start_delay for dsi

This should fix the boot oops for dsi

v2:
* Fix indent (Manasi)
v3:
* Remove redundant condition (Matt Roper)

Fixes: 4e3cdb45 ("drm/i915/dp: Master/Slave enable/disable sequence for bigjoiner")
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201119232615.23231-1-manasi.d.navare@intel.com