drm/ttm: Don't include drm_hashtab.h

Remove the include statement for drm_hashtab.h. It's not required
by TTM.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211129094841.22499-2-tzimmermann@suse.de