drm/i915: Make intel_dsi_init() return void

Functions that can't fail are such a bliss to work with, it'd be shame
to miss the occasion. The "failure" mode is the DSI connector not being
created, the rest of the initialization can carry on happily.

We weren't even checking that value anyway.

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Suggested-by: Shobhit Kumar <shobhit.kumar@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Shobhit Kumar <shobhit.kumar@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Also convert the missed return statement due to other patches
merged meanwhile.]
[danvet2: Squash in fixup from Damien to remove empty return; at the
end of intel_dsi_init.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>