drm/vc4: fix spelling mistake: "dimesions" -> "dimensions"

Trivial fix to spelling mistake in DRM_ERROR message and split
over two lines to clean up a "line over 80 characters" checkpatch
warning.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170514170016.6802-1-colin.king@canonical.com