drm/i915: gen6_enable_rps() wants to be called after ring initialisation

Currently we call gen6_enable_rps() (which writes into the per-ring
register mmio space) from intel_modeset_init_hw() which is called before
we initialise the rings. If we defer intel_modeset_init_hw() until
afterwards (in the intel_modeset_gem_init() phase) all is well.

v2: Rectify ordering of gem vs display HW init upon resume. (Daniel)

v3: Fix up locking. (Paulo)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Smash Paulo's locking fix onto Chris' patch.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>