drm/tegra: dsi: Enhance runtime power management

The MIPI DSI output on Tegra SoCs requires some external logic to
calibrate the MIPI pads before a video signal can be transmitted. This
MIPI calibration logic requires to be powered on while the MIPI pads are
being used, which is currently done as part of the DSI driver's probe
implementation.

This is suboptimal because it will leave the MIPI calibration logic
powered up even if the DSI output is never used.

On Tegra114 and earlier this behaviour also causes the driver to hang
while trying to power up the MIPI calibration logic because the power
partition that contains the MIPI calibration logic will be powered on
by the display controller at output pipeline configuration time. Thus
the power up sequence for the MIPI calibration logic happens before
it's power partition is guaranteed to be enabled.

Fix this by splitting up the API into a request/free pair of functions
that manage the runtime dependency between the DSI and the calibration
modules (no registers are accessed) and a set of enable, calibrate and
disable functions that program the MIPI calibration logic at points in
time where the power partition is really enabled.

While at it, make sure that the runtime power management also works in
ganged mode, which is currently also broken.

Reported-by: Jonathan Hunter <jonathanh@nvidia.com>
Tested-by: Jonathan Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>