drm/sti: Adjust two checks for null pointers in sti_hqvdp_probe()

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The script “checkpatch.pl” pointed information out like the following.

Comparison to NULL could be written !…

Thus fix the affected source code places.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: http://patchwork.freedesktop.org/patch/msgid/918229e6-80fe-629f-8fec-bf60ac15275f@users.sourceforge.net