drm: Use the same mmap-range offset and size for GEM and TTM

GEM defines DRM_FILE_PAGE_OFFSET_{START,SIZE} constants for the
mmap-able range of addresses. TTM can use them as well.

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>