Gallium: Add format PIPE_FORMAT_R8_SRGB

Gallium: Add format PIPE_FORMAT_R8_SRGB

This format is needed to support EXT_texture_sRGB_R8. THe patch adds a new
format enum, the format entries in Gallium and and svga, the mapping between
sRGB and linear formats, and tests.

  v2: - add mapping to linear format for PIPE_FORMATR_R8_SRGB
  v3: - Add texture format to svga format table since otherwise building
        mesa will fail when this driver is enabled. It was not tested
        whether the extension actually works.
  v4: - svga: remove the SVGA specific format definitions and table entries
        and only add correct the location of PIPE_FORMAT_R8_SRGB in the
        format_conversion_table (Ilia Mirkin)
      - Split patch (1/2) to separate Gallium part and mesa/st part.
        (Roland Scheidegger)
      - Trim the commit message to only contain the relevant parts from the
        split.
  v5: - svga: correct location of PIPE_FORMAT_SRGB_R8 (Ilia Mirkin)

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>