drm/amd/display: Simplify handle_hpd_rx_irq()

There is a local reference to the dc_link that wasn't being
used so we shorten references throughout the function.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Acked-by: Andrey Grodzovsky <andey.grodzovsky@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>