drm/i915: Nuke not needed members of dram_info

Valid, ranks and bandwidth_kbps are set into dram_info but are not
used anywhere else so nuking it.

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210128164312.91160-1-jose.souza@intel.com