drm/i915: Protect the argument expansion in LRI and SRM macros

It seems like it wouldn't be too unlikely to be wanting to use a an
expression in the macro argument and things could go very wrong.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>