drm/i915/crc: implement get_crc_sources callback

This patch implements get_crc_sources callback, which returns list of
all the valid crc sources supported by driver in current platform.

Changes since V1:
 - Return array of crc sources

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180713135942.25061-8-mahesh1.kumar@intel.com