drm/amd/display (gcc13): fix enum mismatch

drm/amd/display (gcc13): fix enum mismatch

commit 545094d993f4639482018becda5f2a47d126f0ab upstream.

rn_vbios_smu_set_dcn_low_power_state() produces a valid warning with
gcc-13:
  drivers/gpu/drm/amd/display/dc/clk_mgr/dcn21/rn_clk_mgr_vbios_smu.c:237:6: error: conflicting types for 'rn_vbios_smu_set_dcn_low_power_state' due to enum/integer mismatch; have 'void(struct clk_mgr_internal *, enum dcn_pwr_state)'
  drivers/gpu/drm/amd/display/dc/clk_mgr/dcn21/rn_clk_mgr_vbios_smu.h:36:6: note: previous declaration of 'rn_vbios_smu_set_dcn_low_power_state' with type 'void(struct clk_mgr_internal *, int)'

I.e. the type of the 2nd parameter of
rn_vbios_smu_set_dcn_low_power_state() in the declaration is int, while
the definition spells enum dcn_pwr_state. Synchronize them to the
latter (and add a forward enum declaration).

Cc: Martin Liska <mliska@suse.cz>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: "Pan, Xinhui" <Xinhui.Pan@amd.com>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>