drm/amd/display: Fix underflow for fused display pipes case

[Why]
Depend on res_pool->res_cap->num_timing_generator to query timing
gernerator information, it would case underflow at the fused display
pipes case.
Due to the res_pool->res_cap->num_timing_generator records default
timing generator resource built in driver, not the current chip.

[How]
Some ASICs would be fused display pipes less than the default setting.
In dcnxx_resource_construct function, driver would obatin real timing
generator count and store it into res_pool->timing_generator_count.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Yi-Ling Chen <Yi-Ling.Chen2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>