i965: Ignore GL_SKIP_DECODE_EXT for textures accessed via texelFetch().

i965: Ignore GL_SKIP_DECODE_EXT for textures accessed via texelFetch().

The GL_EXT_texture_sRGB_decode spec says:

"The conversion of sRGB color space components to linear color space is
 always performed if the texel lookup function is one of the texelFetch
 builtin functions.

 Otherwise, if the texel lookup function is one of the texture builtin
 functions or one of the texture gather functions, the conversion of sRGB
 color space components to linear color space is controlled by the
 TEXTURE_SRGB_DECODE_EXT parameter.

 If the TEXTURE_SRGB_DECODE_EXT parameter is DECODE_EXT, the conversion
 of sRGB color space components to linear color space is performed.

 If the TEXTURE_SRGB_DECODE_EXT parameter is SKIP_DECODE_EXT, the value
 is returned without decoding. However, if the texture is also accessed
 with a texelFetch function, then the result of texture builtin functions
 and/or texture gather functions may be returned with decoding or without
 decoding."

This patch makes i965 force sRGB decoding for any textures accessed via
texelFetch().  If textures are accessed via texelFetch() and a regular
texture access function, this will affect the other ones too - which is
fine - it's undefined according to the last paragraph quoted.

We could make both work, but we'd have to emit multiple SURFACE_STATEs,
and have two binding table sections, like we do for texture gather hacks
on older platforms.

Fixes the following Android O CTS test:
dEQP-GLES31.functional.srgb_texture_decode.skip_decode.srgba8.texel_fetch

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>