drm/amd/display: Remove redundant initialization of variable eng_id

The variable eng_id is being initialized with a value that is never
read, it is being re-assigned on the next statment. The assignment
is redundant and can be removed.

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