drm/vmwgfx: Use VM_PFNMAP instead of VM_MIXEDMAP when possible

For shared, and read-only private mappings of graphics memory, use
VM_PFNMAP instead of VM_MIXEDMAP. This means less accounting overhead
when inserting and removing page-table entries. TTM doesn't do this
by default, since there was a performance problem with book-keeping of
write-combined mappings. Since vmwgfx solely uses cached mappings, that's
not a problem and now that the TTM vm has largely been turned into
helpers, we can use VM_PFNMAP on a per-driver basis

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>