drm/i915: extract i915_gem_shrinker.h from i915_drv.h

It used to be handy that we only had a couple of headers, but over time
i915_drv.h has become unwieldy. Extract declarations to a separate
header file corresponding to the implementation module, clarifying the
modularity of the driver.

Ensure the new header is self-contained, and do so with minimal further
includes, using forward declarations as needed. Include the new header
from i915_drv.h to avoid sprinkling includes all over the place; this
can be changed as a follow-up if necessary.

No functional changes.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b8406f72ce5bfb8863a54003b756ebae8b17c9cb.1565271681.git.jani.nikula@intel.com

drivers/gpu/drm/i915/i915_drv.h

@@ -71,6 +71,7 @@
 #include "display/intel_opregion.h"
 
 #include "gem/i915_gem_context_types.h"
+#include "gem/i915_gem_shrinker.h"
 #include "gem/i915_gem_stolen.h"
 
 #include "gt/intel_lrc.h"
@@ -2436,23 +2437,6 @@ struct drm_i915_gem_object *
 i915_gem_object_create_internal(struct drm_i915_private *dev_priv,
 				phys_addr_t size);
 
-/* i915_gem_shrinker.c */
-unsigned long i915_gem_shrink(struct drm_i915_private *i915,
-			      unsigned long target,
-			      unsigned long *nr_scanned,
-			      unsigned flags);
-#define I915_SHRINK_UNBOUND	BIT(0)
-#define I915_SHRINK_BOUND	BIT(1)
-#define I915_SHRINK_ACTIVE	BIT(2)
-#define I915_SHRINK_VMAPS	BIT(3)
-#define I915_SHRINK_WRITEBACK	BIT(4)
-
-unsigned long i915_gem_shrink_all(struct drm_i915_private *i915);
-void i915_gem_driver_register__shrinker(struct drm_i915_private *i915);
-void i915_gem_driver_unregister__shrinker(struct drm_i915_private *i915);
-void i915_gem_shrinker_taints_mutex(struct drm_i915_private *i915,
-				    struct mutex *mutex);
-
 /* i915_gem_tiling.c */
 static inline bool i915_gem_object_needs_bit17_swizzle(struct drm_i915_gem_object *obj)
 {