ARM: tegra: cpufreq: Take an extra reference to pllx

During cpu frequency changes, take an extra reference to pllx so
that it doesn't turn off and on while the cpu is temporarily on
pllp.  If the cpu is moved to pllp permanently, pllx will be
turned off.

Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Colin Cross <ccross@android.com>