drm/gma500: fix spelling mistake "pannel" -> "panel"

There a handful of spelling mistakes. fix them.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200805124227.20005-1-colin.king@canonical.com