drm/xe: Move rebar to its own file

drm/xe: Move rebar to its own file

Now that xe_pci.c calls the rebar directly, it doesn't make sense to
keep it in xe_vram.c since it's closer to the PCI initialization than to
the VRAM. Move it to its own file.

While at it, add a better comment to document the possible values for
the vram_bar_size module parameter.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patch.msgid.link/20251219211650.1908961-5-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>