drm/cirrus: remove leftover files

cirrus_drv.h and cirrus_ttm.c are unused since commit ab3e023b
("drm/cirrus: rewrite and modernize driver"), apparently I ran "rm"
instead of "git rm" on them so they are still in present the tree.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190522103307.12711-1-kraxel@redhat.com