gpu: host1x: mipi: Preserve the contents of MIPI_CAL_CTRL

By paving the CTRL reg value, the current code changes MIPI_CAL_PRESCALE
("Auto-cal calibration step prescale") from 1us to 0.1us (val=0). In the
description for PHY's noise filter (MIPI_CAL_NOISE_FLT), the TRM states
that if the value of the prescale is 0 (or 0.1us), the filter should be
set between 2-5. However, the current code sets it to 0.

For now, let's keep the prescale and filter values as-is, which is most
likely the power-on-reset values of 0x2 and 0xa, respectively.
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers/gpu/host1x/mipi.c

@@ -208,7 +208,9 @@ int tegra_mipi_calibrate(struct tegra_mipi_device *device)
 		tegra_mipi_writel(device->mipi, value, modules[i].reg);
 	}
 
-	tegra_mipi_writel(device->mipi, MIPI_CAL_CTRL_START, MIPI_CAL_CTRL);
+	value = tegra_mipi_readl(device->mipi, MIPI_CAL_CTRL);
+	value |= MIPI_CAL_CTRL_START;
+	tegra_mipi_writel(device->mipi, value, MIPI_CAL_CTRL);
 
 	err = tegra_mipi_wait(device->mipi);