metag: dma: remove dead code in dma_alloc_init()

Meta has 2 levels of page table so the pmd folds into the pud which
folds into the pgd. Therefore the !pmd check in dma_alloc_init() is dead
code since it essentially checks whether:
  (init_mm->pgd + 0x770) == 0

Remove the check.

Reported-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: James Hogan <james.hogan@imgtec.com>