drm/xe: Add xe_ttm_vram_get_avail

The PF driver will need to know size of the remaining available
VRAM to estimate fair VRAM allocations that could be used across
all VFs in automatic VFs provisioning mode. Add helper function
for that. We will use it in upcoming patch.

Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240415173937.1287-4-michal.wajdeczko@intel.com