drm/st7735r: Use devm_drm_dev_alloc

Already using devm_drm_dev_init, so very simple replacment.

Aside: There was an oddity in the old code, we allocated priv but in
the error path we've freed priv->dbidev ...

Acked-by: David Lechner <david@lechnology.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: David Lechner <david@lechnology.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200415074034.175360-14-daniel.vetter@ffwll.ch