drm/tegra: dsi: Clear enable register if powered by bootloader

In cases where the DSI module is left on by the bootloader
some panels may fail to initialize if the enable register is not cleared
before the panel's initialization sequence is sent, so clear it if that
is the case.

Signed-off-by: Diogo Ivo <diogo.ivo@tecnico.ulisboa.pt>
Signed-off-by: Thierry Reding <treding@nvidia.com>