drm/i915: Hold struct_mutex during i915_save_state/i915_restore_state

Lots of register access in these functions, some of which requires the
struct mutex.

These functions now hold the struct mutex across the calls to
i915_save_display and i915_restore_display, and so the internal mutex
calls in those functions have been removed. To ensure that no-one else
was calling them (and hence violating the new required locking
invarient), those functions have been made static.

gen6_enable_rps locks the struct mutex, and so i915_restore_state
unlocks the mutex around calls to that function.

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Keith Packard <keithp@keithp.com>