drm/amd/display: Clean up PSR helper functions

drm/amd/display: Clean up PSR helper functions

[Why & How]
Use the existing local dc variable in amdgpu_dm_set_psr_caps() instead
of redundantly dereferencing link->ctx->dc.

Simplify amdgpu_dm_psr_is_active_allowed() by replacing with early
return and inlining the intermediate stream variable.

No functional changes.

Assisted-by: Copilot:Claude-Sonnet-4.6
Reviewed-by: Ray Wu <ray.wu@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>