drm/xe/client: Check return value of xe_force_wake_get

xe_force_wake_get() can return error so check it's return value
before reading gpu_timestamp value.

v2: set HWE to NULL instead of setting timestamp to 0(Lucas)
    Add a warn on for xe_force_wake_put(Himal)

Fixes: 188ced1e

 ("drm/xe/client: Print runtime to fdinfo")
Cc: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240625094228.5327-1-nirmoy.das@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>