drm/i915: Extend pcode mailbox interface

In sandybridge_pcode_read and sandybridge_pcode_write,
extend the mbox parameter from u8 to u32.

On Haswell and Sandybridge, bits 7:0 encode the mailbox
command and bits 28:8 are used for address control for
specific commands.

Based on suggestion from Ville Syrjälä.

Signed-off-by: Tom O'Rourke <Tom.O'Rourke@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --cc drivers/gpu/drm/i915/i915_drv.h
Simple merge

diff --cc drivers/gpu/drm/i915/intel_pm.c
Simple merge