drm/tinydrm: Advertise that we can do only DRM_FORMAT_MOD_LINEAR.

Without this, the xserver relies on what the 3D driver exposes and
assumes that the display can handle it, and then the DRM driver
happily tries to scan out a tiled format.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181025162635.6689-1-eric@anholt.net


Acked-by: Noralf Trønnes <noralf@tronnes.org>