uapi/drm/i915_drm.h: fix userspace compilation.

commit 346add78
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Tue Jul 14 18:07:30 2015 +0200

    drm/i915: Use expcitly fixed type in compat32 structs

changed the type of param field in drm_i915_getparam from int to
s32. This header is exported to userspace and needs to use userspace
type __s32 instead.

This fixes userspace compilation errors like the following:
include/drm/i915_drm.h:361:2: error: unknown type name 's32'
  s32 param;

Signed-off-by: Artem Savkov <asavkov@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>