Revert "drm/gem: Rename drm_gem_dumb_map_offset() to drm_gem_map_offset()"

This reverts commit 220df83a

.

Turns out drm_gem_dumb_map_offset really only worked for the dumb buffer
case, so revert the name change.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sean Paul <sean@poorly.run>
Link: https://patchwork.freedesktop.org/patch/msgid/20190807145253.2037-2-sean@poorly.run