drm/i915: have *_debugfs_init() functions return void.

Since commit 987d65d01356 (drm: debugfs: make
drm_debugfs_create_files() never fail), drm_debugfs_create_files() never
fails and should return void. Therefore, remove its use as the
return value of debugfs_init() functions and have the functions return
void.

v2: convert intel_display_debugfs_register() stub to return void too.

Signed-off-by: Wambui Karuga <wambui.karugax@gmail.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200310133121.27913-15-wambui.karugax@gmail.com

drm/i915: split out display debugfs to a separate file

The i915_debugfs.c has grown more than a little unwieldy. Split out the
display related debugfs code to a file of its own under display/,
initialized with a separate call. No functional changes.

v2:
- Also moved i915_frontbuffer_tracking, i915_gem_framebuffer,
i915_power_domain_info, i915_dmc_info, i915_ipc_status (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
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/20200211161451.6867-2-jani.nikula@intel.com

drm/i915: extract i915_debugfs.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
only where needed, and sort the modified include directives while at it
and as needed.

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/2843b028d65e118dc40316aa84bf620a93f6c67b.1556809195.git.jani.nikula@intel.com