drm/i915: fix Makefile sort and indent

Unify the line continuations and indents, and sort the build lists.

Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231026101333.875406-1-jani.nikula@intel.com