r600g: fix up the rules for enabling SOURCE_FORMAT(EXPORT_NORM)

r600g: fix up the rules for enabling SOURCE_FORMAT(EXPORT_NORM)

Setting SOURCE_FORMAT to EXPORT_NORM is an optimization.
Leaving SOURCE_FORMAT at 0 will work in all cases, but is less
efficient.  The conditions for the setting the EXPORT_NORM
optimization are as follows:

R600/RV6xx:
BLEND_CLAMP is enabled
BLEND_FLOAT32 is disabled
11-bit or smaller UNORM/SNORM/SRGB

R7xx/evergreen:
11-bit or smaller UNORM/SNORM/SRGB
16-bit or smaller FLOAT

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>