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