drm/ttm: cleanup coding style and implementation.

Only functional change is to always keep io_reserved_count up to date
for debugging even when it is not used otherwise.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/378242/