drm: Move drm_memory.c map support declarations to <drm/drm_legacy.h>

And replace the drm_core_ prefix with drm_legacy_ since really, this
isn't core stuff.

Also drop drm_core_dropmap since it's unused.

v2: Fix up i810.ko fully which somehow slipped through.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>