gpu: ion: fix sparse non static symbol warnings

Fixes the following sparse warnings:

drivers/staging/android/ion/tegra/tegra_ion.c:23:19: warning:
 symbol 'idev' was not declared. Should it be static?
drivers/staging/android/ion/tegra/tegra_ion.c:24:19: warning:
 symbol 'tegra_user_mapper' was not declared. Should it be static?
drivers/staging/android/ion/tegra/tegra_ion.c:25:5: warning:
 symbol 'num_heaps' was not declared. Should it be static?
drivers/staging/android/ion/tegra/tegra_ion.c:26:17: warning:
 symbol 'heaps' was not declared. Should it be static?
drivers/staging/android/ion/tegra/tegra_ion.c:28:5: warning:
 symbol 'tegra_ion_probe' was not declared. Should it be static?
drivers/staging/android/ion/tegra/tegra_ion.c:66:5: warning:
 symbol 'tegra_ion_remove' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>