drm/amd/display: Remove redundant checks for pipe_ctx->stream

[WHAT & HOW]
The null checks for pipe_ctx->stream and pipe_ctx->stream_res.tg
are redundant as they were already dereferenced previously, as
reported by Coverity; therefore the null checks are removed.

This fixes 6 REVERSE_INULL issues reported by Coverity.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>