anv/image: Add a helper for determining when fast clears are supported

anv/image: Add a helper for determining when fast clears are supported

v2 (Jason Ekstrand):
 - Return an enum instead of a boolean

v3 (Jason Ekstrand):
 - Return ANV_FAST_CLEAR_NONE instead of false (Topi)
 - Rename ANV_FAST_CLEAR_ANY to ANV_FAST_CLEAR_DEFAULT_VALUE
 - Add documentation for the enum values

v4 (Jason Ekstrand):
 - Remove a dead comment

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>