drm/i915/dsi: Parse the I2C element from the VBT MIPI sequence block (v3)

Parsing the i2c element is mainly done to transfer the payload from the
MIPI sequence block to the relevant slave device. In some cases, the
commands that are part of the payload can be used to turn on the backlight.

This patch is actually a refactored version of this old patch:
https://lists.freedesktop.org/archives/intel-gfx/2014-December/056897.html

In addition to the refactoring, the original patch is augmented by
looking up the i2c bus from ACPI NS instead of relying on the bus number
provided in the VBT.

This patch was tested on Aava Mobile's Inari 10 tablet. It enabled
turning on the backlight by transferring the payload to the device.

v2:
- Add DRM_DEV_ERROR for invalid adapter and failed transfer and also
  drop the DRM_DEBUG that existed originally. (Hans)
- Add two gotos instead of one to clean things up properly.

v3:
- Identify the device on which this patch was tested in the commit
  message (Ville)

Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Nabendu Maiti <nabendu.bikash.maiti@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Bob Paauwe <bob.j.paauwe@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200110181123.14536-1-vivek.kasireddy@intel.com