drm/i915: Remove redundant forward declarations from display power headers

Remove the forward declarations for enums and structs from display power
header files that aren't used in prototypes.

Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230616185104.2502003-2-imre.deak@intel.com