drm/amd/display: remove duplicated assignment to grph_obj_type

Variable grph_obj_type is being assigned twice, one of these is
redundant so remove it.

Addresses-Coverity: ("Evaluation order violation")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>