video: tegra: host: gk20a: Fix PMU-less boot

video: tegra: host: gk20a: Fix PMU-less boot

A recent change added proper locking to enabling/disabling ELPG,
however, this patch also assumed that PMU is enabled always. This
patch modifies the code so that we do not crash if PMU is not
configured in.

Bug 1462292

Change-Id: Ia131badb4713a36b2df7def40a38914b65d38da0
Signed-off-by: Arto Merilainen <amerilainen@nvidia.com>
Reviewed-on: http://git-master/r/368732
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Philip Rakity <prakity@nvidia.com>
Tested-by: Philip Rakity <prakity@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: Ken Adams <kadams@nvidia.com>