i965/gen9: Resolve SRGB color buffers when GL_FRAMEBUFFER_SRGB enabled

i965/gen9: Resolve SRGB color buffers when GL_FRAMEBUFFER_SRGB enabled

SKL can't cope with the CCS buffer for SRGB buffers. Normally the
hardware won't see the SRGB formats because when GL_FRAMEBUFFER_SRGB
is disabled these get mapped to their linear equivalents. In order to
avoid relying on the CCS buffer when it is enabled this patch now
makes it flush the renderbuffers.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>