i965/clear: clear_value better precision

i965/clear: clear_value better precision

Test-case with depth-clear 0.5 and format
MESA_FORMAT_Z24_UNORM_X8_UINT fails due inconsistent
clear-value of 0.4999997.
Maybe its better to improve?

CC: Jason Ekstrand <jason.ekstrand@intel.com>
Fixes: 0ae9ce0f29ea (i965/clear: Quantize the depth clear value based on the format)
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111113
Signed-off-by: Sergii Romantsov <sergii.romantsov@globallogic.com>
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>