x86: acpi: Fix calculation of DSDT length

Currently, the calculation for the length of the DSDT table includes any
bytes that are added for alignment, but those bytes are not initialized.

This is because the DSDT length is calculated after a call to
acpi_inc_align(). Split this up into the following sequence:

  * acpi_inc()
  * Calculate DSDT length
  * acpi_align()

Signed-off-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>