drm/omap: fix dmabuf mmap for dma_alloc'ed buffers

omap_gem_dmabuf_mmap() returns an error (with a WARN) when called for a
buffer which is allocated with dma_alloc_*(). This prevents dmabuf mmap
from working on SoCs without DMM, e.g. AM4 and OMAP3.

I could not find any reason for omap_gem_dmabuf_mmap() rejecting such
buffers, and just removing the if() fixes the limitation.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>