drm/i915: Replace DRM_DEBUG with DRM_DEBUG_DRIVER

Replace the DRM_DEBUG with DRM_DEBUG_DRIVER in generic i915 driver.
Then the debug info can be obtained by adding the boot option of
"drm.debug=0x02".

At the same time the debug info in increase/decrease clock is also
printed by using DRM_DEBUG_DRIVER instead of DRM_DEBUG_KMS.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>