drm/ili9486: Use devm_drm_dev_alloc

Already using devm_drm_dev_init, so very simple replacment.

Acked-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Kamlesh Gurudasani <kamlesh.gurudasani@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200415074034.175360-18-daniel.vetter@ffwll.ch