drm/i915: Make read_subslice_reg take engine

The function operates on the render engine so make the input reflect it.

v2:
 * Pass engine to read_subslice_reg. (Chris)
 * Drop inline from read_subslice_reg.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190610125706.26110-1-tvrtko.ursulin@linux.intel.com