drm/i915/dsi: rename platform specific *_exec_gpio() to *_gpio_set_value()

The lowest level functions are about setting GPIO values, not about
executing any sequences anymore.

Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231103201831.1037416-5-andriy.shevchenko@linux.intel.com