Allow specifying a limit on visible VRAM via a module parameter. This is
helpful for testing performance under visible VRAM pressure.

v2: Add cast to 64-bit (Christian König)
Signed-off-by: NJohn Brooks <john@fastquake.com>
Reviewed-by: NMichel Dänzer <michel.daenzer@amd.com>
Reviewed-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NMichel Dänzer <michel.daenzer@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>