drm/tegra: Delete an unnecessary check before the function call "vunmap"

The vunmap() function performs also input parameter validation.
Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/b3fbdcaf-1bda-7ce9-935b-2d716727ec39@users.sourceforge.net