Add PIPE_CAP to determine if the GL extension
'GL_ARB_framebuffer_no_attachments' shall be
supported.

The driver is required to support 'PIPE_FORMAT_NONE'
via its 'is_format_supported()' callback in order
to determine the MSAA modes the hardware supports so
that values requested from the application using
'GL_ARB_framebuffer_no_attachments' may be quantized
to what the hardware expects.

V.2:
 Fix doc for a more detailed description of the PIPE_CAP
 and the corresponding GL constant.

V.3:
 Renamed and repurposed once again.

V.4:
 Remove CAP from cap_mapping array.

[airlied: fix damaged whitespace]

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>