video: tegra: host: Fix leak in 3D register read

When 3D register is read, and we need to do a context switch, the old
context reference is incremented, but it never gets decremented. Fix
by using nvhost_job_get_hwctx() to acquire the reference.

Bug 1031746

Change-Id: Ic72637cb2399f960e2297733bfe45e585a8ec180
Signed-off-by: Terje Bergstrom <tbergstrom@nvidia.com>
Reviewed-on: http://git-master/r/123678
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Arto Merilainen <amerilainen@nvidia.com>
Reviewed-by: Juha Tukkinen <jtukkinen@nvidia.com>
GVS: Gerrit_Virtual_Submit