drm/nouveau/disp: Fix missing backlight control on Macbook 5,1

Macbook 5,1 with MCP79 lost its backlight control since the recent
change for supporting GSP-RM; it rewrote the whole nv50 backlight
control code and each display engine is supposed to have an entry for
IOR bl callback, but it didn't cover mcp77.

This patch adds the missing bl entry initialization for mcp77 display
engine to recover the backlight control.

Fixes: 2274ce7e ("drm/nouveau/disp: add output backlight control methods")
Cc: stable@vger.kernel.org
Link: https://bugzilla.suse.com/show_bug.cgi?id=1223838


Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250102114944.11499-1-tiwai@suse.de