drm/i915/dram: add accessor for struct dram_info and use it

drm/i915/dram: add accessor for struct dram_info and use it

Add a function to get the (const) pointer to struct dram_info, and use
that to obtain the pointer instead of poking at i915->dram_info
directly.

Clean up a couple of local variables while at it.

Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Link: https://lore.kernel.org/r/4174edf649e2f6805dab6fd6ce2ec10f4e5f2498.1748337870.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>