drm/amdgpu/uvd3.1: Fix checking whether UVD firmware keyselect was already set

The PASS_MASK is for a different register not for UVD_FW_START.
It was a mistake to check this flag. It should be enough to check
that the register is non-zero.

Fixes: 889a2cfd889c ("drm/amdgpu/uvd3.1: Don't validate the firmware when already validated")
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Link: https://patch.msgid.link/20261006211607.75407-1-timur.kristof@gmail.com
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 9dee462c83259ca769aa1bf82228adee9831685c)
Cc: stable@vger.kernel.org