drm/vmwgfx: Fix an error return check in vmw_compat_shader_add()

drm/vmwgfx: Fix an error return check in vmw_compat_shader_add()

In vmw_compat_shader_add(), the return value check of vmw_shader_alloc()
is not proper. Modify the check for the return pointer 'res'.

Found by code review and compiled on ubuntu 20.04.

Fixes: 18e4a4669c50 ("drm/vmwgfx: Fix compat shader namespace")
Cc: stable@vger.kernel.org
Signed-off-by: Haoxiang Li <lihaoxiang@isrc.iscas.ac.cn>
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patch.msgid.link/20251224091105.1569464-1-lihaoxiang@isrc.iscas.ac.cn