drm/amd/display: add missing NULL check for DML2

Recently, the driver introduce DML2 for future ASIC support.
But, some ASIC's hubbub pointer is null before calling.
It cause the below null pointer issue, so add null check to fix it.

BUG: kernel NULL pointer dereference, address: 0000000000000000
RIP: 0010:dc_create_resource_pool+0xc1/0x2c0 [amdgpu] Call Trace:
 <TASK>
 ? show_regs.cold+0x1a/0x1f
 ? __die_body+0x20/0x70
 ? __die+0x2b/0x37
 ? page_fault_oops+0x136/0x2c0
 ? do_user_addr_fault+0x303/0x660
 ? exc_page_fault+0x77/0x170
 ? asm_exc_page_fault+0x27/0x30
 ? dc_create_resource_pool+0xc1/0x2c0 [amdgpu]  ? dc_create_resource_pool+0x243/0x2c0 [amdgpu]
 dc_create+0x23f/0x6b0 [amdgpu]
 ? dmi_matches+0xa3/0x200
 amdgpu_dm_init+0x2bd/0x22a0 [amdgpu]

Fixes: 7966f319 ("drm/amd/display: Introduce DML2")
Signed-off-by: Bob Zhou <bob.zhou@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_resource.c

@@ -321,7 +321,7 @@ struct resource_pool *dc_create_resource_pool(struct dc  *dc,
 				res_pool->ref_clocks.xtalin_clock_inKhz;
 			res_pool->ref_clocks.dchub_ref_clock_inKhz =
 				res_pool->ref_clocks.xtalin_clock_inKhz;
-			if ((res_pool->hubbub->funcs->get_dchub_ref_freq))
+			if (res_pool->hubbub && res_pool->hubbub->funcs->get_dchub_ref_freq)
 				res_pool->hubbub->funcs->get_dchub_ref_freq(res_pool->hubbub,
 					res_pool->ref_clocks.dccg_ref_clock_inKhz,
 					&res_pool->ref_clocks.dchub_ref_clock_inKhz);