drm/komeda: Refine function to_d71_input_id

Since the component_state->input[i] is only valid when it is active, the
content of input[i] is undefined if it is inactive. The user must check the
state->active_inputs with input index firstly before using state->input[i].
Refine the function to_d71_input_id and directly move such check into it.
Simplify the using and avoid problem that caller forget the valid check.

Depends on:
- https://patchwork.freedesktop.org/series/60856/

Signed-off-by: James Qian Wang (Arm Technology China) <james.qian.wang@arm.com>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>