drm/ttm: Quick-test mmap offset in ttm_bo_mmap()

A BO's address has to be at least the minimum offset. Sharing this
test in ttm_bo_mmap() removes code from drivers. A full buffer-address
validation is still done within drm_vma_offset_lockup_locked().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>