drm/msm: Fix possible null dereference on failure of get_pages()

Commit 62e3a3e3 changed get_pages() to initialise
msm_gem_object::pages before trying to initialise msm_gem_object::sgt,
so that put_pages() would properly clean up pages in the failure
case.

However, this means that put_pages() now needs to check that
msm_gem_object::sgt is not null before trying to clean it up, and
this check was only applied to part of the cleanup code.  Move
it all into the conditional block.  (Strictly speaking we don't
need to make the kfree() conditional, but since we can't avoid
checking for null ourselves we may as well do so.)

Fixes: 62e3a3e3 ("drm/msm: fix leak in failed get_pages")
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>