gallium: add hook on getting canonical format

gallium: add hook on getting canonical format

On swizzled copies canonical formats are used to reduce the formats to a
simpler subset.

Nevertheless, it is possible that some of the canonical formats defined
in Gallium are actually not supported by the drivers themselves.

This provides a driver-defined hook that can be used to provide an
alternative canonical format in case the canonical one defined by
Gallium is not supported by the driver.

Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15693>