drm/amd/display: Fix NULL pointer assumptions in dcn42_init_hw()

drm/amd/display: Fix NULL pointer assumptions in dcn42_init_hw()

dcn42_init_hw() calls update_bw_bounding_box() when FAMS2 is disabled or
when the dchub reference clock changes. However the existing condition
mixes the callback pointer check with only one side of the || expression:

  ((!fams2_enable && update_bw_bounding_box) || freq_changed)

This allows the block to be entered through the freq_changed path even
when update_bw_bounding_box() is NULL. The function is then called
unconditionally inside the block, which can lead to a NULL pointer
dereference.

Additionally, the code dereferences dc->clk_mgr->bw_params without
verifying that dc->clk_mgr and bw_params are valid.

Restructure the condition so that the update trigger remains the same
(FAMS2 disabled or dchub ref clock changed), but guard the call with
explicit checks for:

  - update_bw_bounding_box callback
  - dc->clk_mgr
  - dc->clk_mgr->bw_params

Also introduce a helper boolean (dchub_ref_freq_changed) to improve
readability of the clock-change condition.

This fixes Smatch warnings about inconsistent NULL assumptions in
dcn42_init_hw().

drivers/gpu/drm/amd/amdgpu/../display/dc/hwss/dcn42/dcn42_hwseq.c:264 dcn42_init_hw() error: we previously assumed 'dc->clk_mgr' could be null (see line 253)
drivers/gpu/drm/amd/amdgpu/../display/dc/hwss/dcn42/dcn42_hwseq.c:278 dcn42_init_hw() error: we previously assumed 'dc->res_pool->funcs->update_bw_bounding_box' could be null (see line 274)

Cc: Roman Li <roman.li@amd.com>
Cc: Alex Hung <alex.hung@amd.com>
Cc: Jerry Zuo <jerry.zuo@amd.com>
Cc: Sun peng Li <sunpeng.li@amd.com>
Cc: Tom Chung <chiahsuan.chung@amd.com>
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>