drm/i915: Add missing forward declarations/includes to display power headers

Add the seq_file struct forward declaration to intel_display_power.h
fixing the build error below. While at it add the rest of missing
forward declarations/includes to the display power header files.

In file included from <command-line>:
./../drivers/gpu/drm/i915/display/intel_display_power.h:255:70: error: 'struct seq_file'
declared inside parameter list will not be visible outside of this definition or declaration [-Werror]
  255 | void intel_display_power_debug(struct drm_i915_private *i915, struct seq_file *m);
      |                                                                      ^~~~~~~~

Closes: https://lore.kernel.org/intel-gfx/89adc1ac-25a0-6eb6-4cc9-ab6cc8d49730@infradead.org/
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230616185104.2502003-1-imre.deak@intel.com
(cherry picked from commit f4fab137

)
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>