drm/amd/display: Remove plane_res.mi check in dce110_apply_ctx_for_surface

plane_res.mi (memory interface) can never be NULL for DCE110

Found by smatch:
drivers/gpu/drm/amd/amdgpu/../display/dc/dce110/dce110_hw_sequencer.c:2881
dce110_apply_ctx_for_surface() error: we previously assumed
'pipe_ctx->plane_res.mi' could be null (see line 2873)

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>