drm/amd/display: invalid parameter check in dmub_hpd_callback

The function performs a check on the "adev" input parameter, however, it
is used before the check.

Initialize the "dev" variable after the sanity check to avoid a possible
NULL pointer dereference.

Fixes: e27c41d5 ("drm/amd/display: Support for DMUB HPD interrupt handling")
Addresses-Coverity-ID: 1493909 ("Null pointer dereference")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>