drm/amd/display: return status of dmub_srv_get_fw_boot_status

gcc with W=1 reports
drivers/gpu/drm/amd/amdgpu/../display/dc/dc_dmub_srv.c:
  In function ‘dc_dmub_srv_optimized_init_done’:
drivers/gpu/drm/amd/amdgpu/../display/dc/dc_dmub_srv.c:184:26:
  error: variable ‘dmub’ set but not used [-Werror=unused-but-set-variable]
  184 |         struct dmub_srv *dmub;
      |                          ^~~~

The return status is never set.
It looks like a call to dmub_srv_get_fw_boot_status is missing.

Fixes: 499e4b1c ("drm/amd/display: add mechanism to skip DCN init")
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>