drm/amdkfd: Clean up inline handling

- inline functions need to be static inline, otherwise gcc can opt to
  not inline and the linker gets unhappy.
- no forward decls for inline functions, just include the right headers.

Cc: Oded Gabbay <oded.gabbay@gmail.com>
Cc: Ben Goz <ben.goz@amd.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1466500235-21282-2-git-send-email-daniel.vetter@ffwll.ch