drm/amd/display: Remove redundant checks in set_default_colors

pipe_ctx->stream and pipe_ctx->plane_state are never NULL

Found by smatch:
drivers/gpu/drm/amd/amdgpu/../display/dc/dce110/dce110_hw_sequencer.c:2111
set_default_colors() error: we previously assumed 'pipe_ctx->stream'
could be null (see line 2101)

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>