drm/i915/dsi: reduce tedious repetition

Make it a bit tidier and safer.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/cab24a84811ddbae72d8c3a5f59d29f57b1d3aad.1450702954.git.jani.nikula@intel.com