drm/i915: Clean up intel_get_dram_info() a bit

Remove the pointless zero initialization of bunch of things
(the thing is kzalloc()ed).

Also throw out the mostly useless on-stack string. I think
it'll be clear enough from the logs that 0 means unknown.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190306203551.24592-11-ville.syrjala@linux.intel.com