drm/tidss: Use devm_drm_dev_alloc

Already using devm_drm_dev_init, so very simple replacment.

Tested-by: Jyri Sarha <jsarha@ti.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jyri Sarha <jsarha@ti.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200415074034.175360-24-daniel.vetter@ffwll.ch