drm/amdgpu: limit visible vram if it's smaller than the BAR

drm/amdgpu: limit visible vram if it's smaller than the BAR

BugLink: http://bugs.launchpad.net/bugs/1546572
In some cases the amount of vram may be less than the BAR size,
if so, limit visible vram to the amount of actual vram, not the
BAR size.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit a1493cd575678910f90d488069a8a2c41a144fda)
Signed-off-by: Alberto Milone <alberto.milone@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>