drm/amd/pm: remove unneeded variable

fix the following coccicheck warning:

drivers/gpu/drm/amd/pm/swsmu/smu11/navi10_ppt.c:1657:14-18: Unneeded variable: "size".

Signed-off-by: Mingtong Bao <baomingtong001@208suo.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>