drm/i915: move the edram detection out of uncore init

edram is not part of uncore and there is no requirement for the
detection to be done before we initialize the uncore functions. The
first check on HAS_EDRAM is in the ggtt_init path, so move it to
i915_driver_init_hw, where other dram-related detection happens.

While at it, save the size in MB instead of the capabilities because the
size is the only thing we look at outside of the init function.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190328174533.31532-1-daniele.ceraolospurio@intel.com