drm/i915/dsi: Replace poking of VLV GPIOs behind the driver's back

It's a dirty hack in the driver that pokes GPIO registers behind
the driver's back. Moreoever it might be problematic as simultaneous
I/O may hang the system, see the commit 40ecab55 ("pinctrl:
baytrail: Really serialize all register accesses") for the details.
Taking all this into consideration replace the hack with proper
GPIO APIs being used.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231103201831.1037416-13-andriy.shevchenko@linux.intel.com