drm/i915: vlv: reserve GT power context early

We reserve the space for the power context in stolen memory at a fixed
address from a delayed work. This races with the subsequent driver
init/resume code which could allocate something at that address, so the
reservation for the power context fails. Reserve the space up-front, so
this can't happen. This also adds a missing struct_mutex lock around the
stolen allocation, which wasn't taken in the delayed work path.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: stable@vger.kernel.org
Signed-off-by: Jani Nikula <jani.nikula@intel.com>