drm/i915/dram: hide the dram structs better

Finish the job started in d28ae3b2 ("drm/i915: split out
intel_dram.[ch] from i915_drv.c") by moving struct dram_dimm_info and
dram_channel_info inside intel_dram.c, the only user of the structs.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200227145359.17543-1-jani.nikula@intel.com