drm/xe: Remove stale declaration of xe_mmio_probe_vram()

The declaration of xe_mmio_probe_vram() became useless since
commit 638d1c79 ("drm/xe: Promote VRAM initialization function to
own file"). Remove it.

Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240722190524.71548-2-gustavo.sousa@intel.com