drm/i915: Include the debugfs params header for its own definition

drivers/gpu/drm/i915/i915_debugfs_params.c:228:15: warning: symbol 'i915_debugfs_params' was not declared. Should it be static?
drivers/gpu/drm/i915/i915_debugfs_params.c:228:16: error: no previous prototype for ‘i915_debugfs_params’ [-Werror=missing-prototypes]
  228 | struct dentry *i915_debugfs_params(struct drm_i915_private *i915)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Fixes: c43c5a88 ("drm/i915/params: add i915 parameters to debugfs")
Link: https://patchwork.freedesktop.org/patch/msgid/20200117102145.2948244-1-chris@chris-wilson.co.uk