drm: sti: fix spelling mistake: rejec -> rejection

In other places of the driver the string hdmi_rejection_pll is
used instead of the truncated hdmi_rejec_pll, so use this string
instead to be consistent.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Philippe Cornu <philippe.cornu@foss.st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191004082706.26478-1-colin.king@canonical.com