drm/amd/display: Add missing NULL pointer check within dpcd_extend_address_range

[Why & How]
ASSERT if return NULL from kcalloc.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>