drm/omap: gem: Clean up GEM objects memory flags

The driver assumes that only objects backed by shmem need to be mapped
through DMM. While this is true with the current code, the assumption
won't hold with dma_buf import support.

Condition the mapping based on whether the buffer has been allocated
using the DMA mapping API instead and clean up the flags to avoid having
to check both flags and GEM object filp field to decide how to process
buffers. Flags are not the authoritative source of information regarding
where the buffer memory comes from, and are renamed to make that
clearer.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>