drm/i915/guc: Add support for define guc_log_size in megabytes.

At this moment we can define GuC logs sizes only using pages.
But GuC also allows use for this values expressed in megabytes.
Lets add support for define guc_log_size in megabytes when we
debug of GuC.

v2:
- change buffers size to more friendly (Michał Wajdeczko)
- merge statements in guc_ctl_log_params_flags() (Michał Wajdeczko)
v3:
- fix ifdef (rename DRM_I915_DEBUG_GUC to CONFIG_DRM_I915_DEBUG_GUC)
(Michał Wajdeczko)
- use SZ_* macros to define buffers sizes (Michał Wajdeczko)

Signed-off-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180605151330.9954-2-piotr.piorkowski@intel.com