drm/i915: Rewrite ABS_DIFF() in a safer manner

The new version of the macro does a few things better:
  - protect the arguments,
  - only evaluate the arguments once,
  - check that the arguments are of the same type,

Change LC_FREQ_2K to be a unsigned 64bit constant and removed the '()'
from the caller as a result.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>