drm/vmwgfx: Fix a null-ptr access in the cursor snooper

drm/vmwgfx: Fix a null-ptr access in the cursor snooper

Check that the resource which is converted to a surface exists before
trying to use the cursor snooper on it.

vmw_cmd_res_check allows explicit invalid (SVGA3D_INVALID_ID) identifiers
because some svga commands accept SVGA3D_INVALID_ID to mean "no surface",
unfortunately functions that accept the actual surfaces as objects might
(and in case of the cursor snooper, do not) be able to handle null
objects. Make sure that we validate not only the identifier (via the
vmw_cmd_res_check) but also check that the actual resource exists before
trying to do something with it.

Fixes unchecked null-ptr reference in the snooping code.

Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Fixes: c0951b797e7d ("drm/vmwgfx: Refactor resource management")
Reported-by: Kuzey Arda Bulut <kuzeyardabulut@gmail.com>
Cc: Broadcom internal kernel review list <bcm-kernel-feedback-list@broadcom.com>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Ian Forbes <ian.forbes@broadcom.com>
Link: https://lore.kernel.org/r/20250917153655.1968583-1-zack.rusin@broadcom.com