drm/omapdrm: Drop dma_buf->k(un)map

No in-tree users left.

Note that this is one of the few (if only) implementations of dma-buf
that provided a kmap, but not a vmap implemenation. Given that the
only real user (in-tree at least) of kmap was tegra, and it's
impossible to buy a chip with tegra host1x and ompadrm on the same
SoC, there's no problem here.

Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191118103536.17675-8-daniel.vetter@ffwll.ch