drm/amd/display: Drop unnecessary ret variable for enable_assr()

[Why]
enable_assr() has a res variable that only is changed in one block with
no cleanup necessary.

[How]
Remove variable and return early from failure cases.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>