drm/vkms: Fix vmap_count increment position

Move vmap_count out of the conditional statement since it needs
to be updated for every successful call to vkms_gem_vmap.

Signed-off-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Fixes: bb112b14 ("drm/vkms: Add functions to map/unmap GEM backing storage")
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180801090807.GA4611@haneenDRM