x86: Fix DRAM bank size init with generic board

The intention of the memory init code is that it should work the same with
CONFIG_SYS_GENERIC_BOARD and without. This is tricky because dram_init()
is called prior to relocation with generic board (matching other archs)
and after relocation without generic board.

Adjust the init sequence so that dram_init() is not called in the generic
board case, which seems like the easiest fix for now. Also ensure that
relocation addresses are still calculated.

Signed-off-by: Simon Glass <sjg@chromium.org>