drm/i915: Make RING_PDP relative to engine->mmio_base

The PDP registers are an oddity inside the set of context saved
registers in that they take the engine as a parameter to the macro and
not the mmio_base as the others do. Make it accept the engine->mmio_base
for consistency in programming the context registers.

add/remove: 0/0 grow/shrink: 2/1 up/down: 3/-32 (-29)
Function                                     old     new   delta
emit_ppgtt_update                            324     326      +2
capture                                     5102    5103      +1
execlists_init_reg_state.isra               1128    1096     -32

And similar savings later!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190405123831.9724-1-chris@chris-wilson.co.uk