drm/i915/dsi: Control panel and backlight enable GPIOs on BYT

On Bay Trail devices the MIPI power on/off sequences for DSI LCD panels
do not control the LCD panel- and backlight-enable GPIOs. So far, when
the VBT indicates we should use the SoC for backlight control, we have
been relying on these GPIOs being configured as output and driven high by
the Video BIOS (GOP) when it initializes the panel.

This does not work when the device is booted with a HDMI monitor connected
as then the GOP will initialize the HDMI instead of the panel, leaving the
panel black, even though the i915 driver tries to output an image to it.

Likewise on some device-models when the GOP does not initialize the DSI
panel it also leaves the mux of the PWM0 pin in generic GPIO mode instead
of muxing it to the PWM controller.

This commit makes the DSI code control the SoC GPIOs for panel- and
backlight-enable on BYT, when the VBT indicates the SoC should be used

for backlight control. It also ensures that the PWM0 pin is muxed to the
PWM controller in this case.

This fixes the LCD panel not lighting up on various devices when booted
with a HDMI monitor connected. This has been tested to fix this on the
following devices:

Peaq C1010
Point of View MOBII TAB-P800W
Point of View MOBII TAB-P1005W
Terra Pad 1061
Yours Y8W81

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191216205122.1850923-6-hdegoede@redhat.com