drm/legacy: Fix type for drm_local_map.offset

drm_local_map.offset is not only used for resource_size_t but also
dma_addr_t which may be of different sizes.

Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Fixes: 8e4ff9b5

 ("drm: Remove the dma_alloc_coherent wrapper for internal usage")
Tested-by: Nathan Chancellor <natechancellor@gmail.com> # build
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dave Airlie <airlied@gmail.com>
Cc: Nathan Chancellor <natechancellor@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200402215926.30714-1-chris@chris-wilson.co.uk