drm/i915/dram: use intel_uncore_*() functions for register access

The implicit "dev_priv" local variable use has been a long-standing pain
point in the register access macros I915_READ(), I915_WRITE(),
POSTING_READ(), I915_READ_FW(), and I915_WRITE_FW().

Replace them with the corresponding uncore register accessors
intel_uncore_read(), intel_uncore_write(), intel_uncore_posting_read(),
intel_uncore_read_fw(), and intel_uncore_write_fw().

Rename dev_priv to i915 while at it.

No functional changes.

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/20200225111509.21879-2-jani.nikula@intel.com