mesa/gles: Allow format GL_RED to be used with MESA_FORMAT_R_UNORM

mesa/gles: Allow format GL_RED to be used with MESA_FORMAT_R_UNORM

If the bound framebuffer has a format of MESA_FORMAT_R_UNORM, then
IMPLEMENTATION_COLOR_READ_FORMAT will return GL_RED. This change
applies to OpenGLES contexts where additional restrictions are placed
on the formats that are allowed to be supported.

Fixes OpenGLES 3.1 CTS tests:
 * ES31-CTS.texture_border_clamp.sampling_texture.Texture2DDC16
 * ES31-CTS.texture_border_clamp.sampling_texture.Texture2DDC16Linear
 * ES31-CTS.texture_border_clamp.sampling_texture.Texture2DDC32F
 * ES31-CTS.texture_border_clamp.sampling_texture.Texture2DDC32FLinear

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>