drm/amd/pm: use sysfs_streq for string matching in amdgpu_pm

drm/amd/pm: use sysfs_streq for string matching in amdgpu_pm

The driver uses strncmp() to compare sysfs attribute strings,
which does not handle trailing newlines and lacks NULL safety.

sysfs_streq() is the recommended function for sysfs string equality
checks in the kernel, providing safer and more correct behavior.

replace strncmp() with sysfs_streq() in drivers/gpu/drm/amd/pm/amdgpu_pm.c

Signed-off-by: Yang Wang <kevinyang.wang@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>