drm/xe/nvm: Defer xe->nvm assignment until init succeeds

drm/xe/nvm: Defer xe->nvm assignment until init succeeds

Allocate and initialize the NVM structure using a local pointer and
assign it to xe->nvm only after all initialization steps succeed.

This avoids exposing a partially initialized xe->nvm and removes the
need to explicitly clear xe->nvm on error paths, simplifying error
handling and making the lifetime rules clearer.

Cc: Alexander Usyskin <alexander.usyskin@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Brian Nguyen <brian3.nguyen@intel.com>
Signed-off-by: Shuicheng Lin <shuicheng.lin@intel.com>
Reviewed-by: Brian Nguyen <brian3.nguyen@intel.com>
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patch.msgid.link/20260120183239.2966782-8-shuicheng.lin@intel.com