i965/dri: Handle the linear fb modifier

i965/dri: Handle the linear fb modifier

At image creation create a path for dealing with the linear modifier.
This works exactly like the old usage flags where __DRI_IMAGE_USE_LINEAR
was specified.

During development of this patch series, it was decided that a lack of
modifier was an insufficient way to express the required modifiers. As a
result, 0 was repurposed to mean a modifier for a LINEAR layout.

NOTE: This patch was added for v3 of the patch series.

v2: Rework the algorithm for modifier selection to go from a bitmask
based selection to this priority value.

v3: Make DRM_FORMAT_MOD_INVALID allowed at selection as a way of
identifying no modifiers found (because 0 is LINEAR) (Jason)

v4: Remove the logic to prune unknown modifiers (like those from other
vendors) and simply handle is in select_best_modifier (Jason)

Requested-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>