drm/i915/uc: Create intel_uc_init_mmio

drm/i915/uc: Create intel_uc_init_mmio

This patch adds new function intel_uc_init_mmio which will initialize
MMIO access related variables prior to uc load/init.

v2: Removed unnecessary export of guc_send_init_regs. Created
intel_uc_init_mmio that currently wraps guc_init_send_regs. (Michal)

v3 (Michal): add kerneldoc (Joonas)

Signed-off-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171004153327.32608-4-michal.wajdeczko@intel.com