staging: sm750fb: Replace NULL comparison.

Replaced NULL comparison with pCurrentDviCtrl->pfnInit. This patch fixes
the following checkpatch issue:

CHECK: Comparison to NULL could be written "pCurrentDviCtrl->pfnInit"

Signed-off-by: Varsha Rao <rvarsha016@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>