drm/tegra: dsi: Do not manage clock on enable/disable

In preparation for supporting command mode panels, don't disable the
clock when the output is disabled. The output will be enabled only after
the panel has been programmed in command mode, so the clock must always
remain on.

As a side-effect, pad calibration now only needs to be done at driver
probe time, since neither power nor controller state will go away before
driver removal. While at it, use a 32-bit variable to store register
content because the registers are 32-bit even on 64-bit Tegra.

Signed-off-by: Thierry Reding <treding@nvidia.com>