gallium: Split out PIPE_CAP_TEXTURE_MIRROR_CLAMP_TO_EDGE.

gallium: Split out PIPE_CAP_TEXTURE_MIRROR_CLAMP_TO_EDGE.

Some hardware can do PIPE_TEX_WRAP_MIRROR_REPEAT but not
PIPE_TEX_WRAP_MIRROR_CLAMP and PIPE_TEX_WRAP_MIRROR_CLAMP_TO_BORDER.

Drivers for such hardware would like to advertise support for
ARB_texture_mirror_clamp_to_edge but not EXT_texture_mirror_clamp.

This commit adds a new PIPE_CAP_TEXTURE_MIRROR_CLAMP_TO_EDGE bit,
changes the extension enable to be based on that, and enables it
in all upstream drivers which supported PIPE_CAP_TEXTURE_MIRROR_CLAMP
(so they continue supporting this mode).