drm/i915/dsi: Lookup the i2c bus from ACPI NS only if CONFIG_ACPI=y (v2)

Perform the i2c bus/adapter lookup from ACPI Namespace only if ACPI is
enabled in the kernel config. If ACPI is not enabled or if the lookup
fails, we'll fallback to using the VBT for identifying the i2c bus.

v2: Add fixes tag (Jani)

Fixes: 8cbf89db ("drm/i915/dsi: Parse the I2C element from the VBT MIPI sequence block (v3)")
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>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200115012305.27395-1-vivek.kasireddy@intel.com