drm/vmwgfx: Fix depth input into drm_mode_legacy_fb_format

Currently the pitch is passed in as depth.  This causes
drm_mode_legacy_fb_format() to return the wrong pixel format.

The wrong pixel format will be rejected by vmw_kms_new_framebuffer(),
thus leaving par->set_fb to NULL.

This eventually causes a crash in vmw_fb_setcolreg() when the code
tries to dereference par->set_fb.

Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>