drm/vc4: Fix spelling mistake "mmaping" -> "mmapping"

There are a couple of spelling mistakes in DRM_DEBUG messages. Fix them.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20221021084035.65367-1-colin.i.king@gmail.com