OMAPDSS: HDMI: Fix possible NULL reference

Commit 5391e87d (OMAPDSS: remove dispc's dependency to VENC/HDMI)
introduced a possible NULL reference bug in the HDMI driver when setting
timings. In practice the bug shouldn't happen, as the timings have been
verified earlier, and thus the timings should always be ok.

Fix the possible issue by moving the use of the timings pointer inside
the NULL check.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>