dml/dcn20_fpu file centralizes all DCN2x functions that require FPU access.
Therefore, this patch moves FPU-related code from dcn21 to dcn20_fpu. These
include:
- dcn21_populate_dml_pipes_from_context()
- dcn21_validate_bandwidth_fp() and related:
  - dcn21_calculate_wm(),
  - patch_bounding_box(),
  - calculate_wm_set_for_vlevel()
- renaming update_bw_bounding_box() to dcn21_update_bw_bounding_box(), move
to dcn20_fpu with related static function construct_low_pstate_lvl()

Also, make dcn21_fast_validate_bw() public in dcn21_resource as it is called
by dcn21_validate_bandwidth_fp() now in dcn20_fpu.

Reuse dcn20_fpu_adjust_dppclk() in dcn21_fast_validate_bw() as it isolates
the same FPU operation.

Include dchubbub.h as it is required in dcn21_populate_dml_pipes_from_context()
Signed-off-by: Melissa Wen <mwen@igalia.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>