drm/vmwgfx: Add a new define for vmwgfx user-space debugging

Error messages or debugging message reported during user-space command
submission should not be printed to dmesg by default. So add a new
preprocessor define called VMW_DEBUG_USER which translates to
DRM_DEBUG_DRIVER.

v2: Use VMW_DEBUG_USER instead of using DRM_DEBUG_DRIVER directly.

Signed-off-by: Deepak Rawat <drawat@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>