drm/i915: Fix tabs vs. spaces in sprite code

The sprite code has a bunch of spaces where tabs should be used. Fix it
up.

v2: Make the patch subject more specific (Jani)
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180530165933.11424-3-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>