drm/i915: Add 'offset' to uncore funcs

Add 'u32 offset' to the uncore register access functions. For now
it's the same as 'reg', but once type safety gets added 'reg' will be
the type safe register variable and 'offset' the raw offset.

v2: s/uint32_t/u32/ (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446839236-20035-1-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>