drm/i915: use REG_BIT on FW_BLC_SELF_* macros

drm/i915: use REG_BIT on FW_BLC_SELF_* macros

Use REG_BIT() instead of open coding the shift in the FW_BLC_SELF_*
macro definitions to avoid potentially typing them as 'int'.

For example, this happens when we pass them to _MASKED_BIT_ENABLE(),
because of the typeof() construct there.  When we pass 1 << 15 (the
FW_BLC_SELF_EN macro), we get typeof(1 << 15), which is 'int'.  Then
the value becomes negative (-2147450880) and we try to assign it to a
'u32'.

In practice this is not a problem though, because when we try to
assign -2147450880 to the u32, that becomes 0x80008000, which was the
intended result.

Link: https://lore.kernel.org/intel-gfx/20250827111109.401604-1-luciano.coelho@intel.com/
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>