drm/amd/display: Correct `DMUB_FW_VERSION` macro

The `DMUB_FW_VERSION` macro has a mistake in that the revision field
is off by one byte. The last byte is typically used for other purposes
and not a revision.

Cc: stable@vger.kernel.org
Cc: Sean Wang <sean.ns.wang@amd.com>
Cc: Marc Rossi <Marc.Rossi@amd.com>
Cc: Hamza Mahfooz <Hamza.Mahfooz@amd.com>
Cc: Tsung-hua (Ryan) Lin <Tsung-hua.Lin@amd.com>
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>