winsys/radeon: add env var to disable VM on Cayman/Trinity
winsys/radeon: add env var to disable VM on Cayman/Trinity

Set env var RADEON_VA=0 to disable VM on Cayman/Trinity.
Useful for debugging.

Note: this is a candidate for the 9.1 branch.

Signed-off-by: Alex Deucher <08dc22c6156113f2deff178e35e3ed9b24d6af9e@amd.com>
Reviewed-by: Tom Stellard <85590917cb30a670bf23f5b4273617f8815d1e87@amd.com>
Reviewed-by: Marek Olšák <74bac9a67a04041c3586cda013cc5a11352e67be@gmail.com>

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