drm/amd/display: Drop unnecessary 'rc' variable in amdgpu_dm_backlight_get_level()

drm/amd/display: Drop unnecessary 'rc' variable in amdgpu_dm_backlight_get_level()

[Why]
amdgpu_dm_backlight_get_level() returns a boolean value but is assigned
to a variable named 'rc' which is generally used for return codes.

This can be confusing while looking at the code for other issues.

[How]
Drop the variable and just look directly at the return value of
amdgpu_dm_backlight_get_level() in the if statement.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>