drm/drm_pci.c: Use dma_zalloc_coherent

Replace dma_alloc_coherent + memset with dma_zalloc_coherent.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181023040222.GA1540@jordon-HP-15-Notebook-PC