drm/i915/dsi: update reset and power sequences in panel prepare/unprepare hooks

Based on the documentation alone, it's anyone's guess when exactly we
should be running these sequences. Add power on/off sequences where they
feel logical and update assert/deassert reset. The drm panel hooks don't
currently offer us more granularity anyway.

v2: update assert/deassert reset as well (Ville)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1543320494df953fa073e136248238eaa1eed059.1474286487.git.jani.nikula@intel.com