drm/i915: Send DPI command explicitely in LP mode

Though HS mode also should work.

v2: Change parameter as "bool hs" as suggested by Jani

Signed-off-by: Shobhit Kumar <shobhit.kumar@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>