drm/i915: split out intel_dram.[ch] from i915_drv.c

The DRAM related routines are pretty isolated from the rest of the
i915_drv.c, split it out to a separate file. Put the eDRAM stuff in the
same bag, and rename the visible functions to have intel_dram_
prefix. Do some benign whitespace fixes and dev_priv -> i915 conversions
while at it.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200225111509.21879-1-jani.nikula@intel.com