drm/i915: refactor debugfs create functions

All r/w debugfs files are created equal.

v2: Add some newlines to make the code easier on the eyes as requested
by Ben Widawsky.

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>