drm/ttm: Define a single DRM_FILE_PAGE_OFFSET constant

Most TTM drivers define the constant DRM_FILE_PAGE_OFFSET of the same
value. The only exception is vboxvideo, which is being converted to the
new offset by this patch. Unifying the constants in a single place
simplifies the driver code.

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>