drm/omap: remove now unused functions

Some functions are unused after removal of the kmap_atomic
DMA-buf interface.

Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: f664a526

 ("dma-buf: remove kmap_atomic interface")
Link: https://patchwork.freedesktop.org/series/45245/
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>