drm: Remove defunct dma_buf_kmap stubs

drm: Remove defunct dma_buf_kmap stubs

Since commit 09ea0dfbf972 ("dma-buf: make map_atomic and map function
pointers optional"), we no longer need to provide stub no-op functions
as the core now provides them directly.

References: 09ea0dfbf972 ("dma-buf: make map_atomic and map function pointers optional")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180807174748.4503-1-chris@chris-wilson.co.uk