drm/amd/display: Rename backlight_properties to pwr_backlight_properties

drm/amd/display: Rename backlight_properties to pwr_backlight_properties

[Why]
'struct backlight_properties' in power.c has the same name as the kernel's
struct defined in <linux/backlight.h>. In out-of-tree backport build
environments, the header is forcefully injected via command-line includes,
causing a redefinition error.

[How]
Rename the file-local 'struct backlight_properties' to
'pwr_backlight_properties' to avoid the name collision. No functional
change.

Reviewed-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Signed-off-by: James Lin <pinglei.lin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>