drm/amdgpu/vce3: Fix VCE 3 firmware size and offsets

drm/amdgpu/vce3: Fix VCE 3 firmware size and offsets

The VCPU BO contains the actual FW at an offset, but
it was not calculated into the VCPU BO size.
Subtract this from the FW size to make sure there is
no out of bounds access.

This may fix VM faults when using VCE 3.

Cc: John Olender <john.olender@gmail.com>
Fixes: e98226221467 ("drm/amdgpu: recalculate VCE firmware BO size")
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 15c369257bd85f47a514744f960c5a51c867716f)