drm/i915/drv: use intel_uncore_write() 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 the sole remaining I915_WRITE() in i915_drv.c with
intel_uncore_write(), although it might be better to keep the entire
file void of direct register access.

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/20200225111509.21879-3-jani.nikula@intel.com