gma: Make PCI config access mandatory

We always have access to the PCI device now. So we can drop the old
`Check_Platform ()` that relied on the MMIO space only.  Some paths
also get simplified, as we don't need the `PCI_Usable` flag anymore.

Change-Id: I29f2d79484eae2f84b8547e52869d8f6246024a0
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.sourcearcade.org/c/libgfxinit/+/537
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Thomas Heijligen <src@posteo.de>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
diff --git a/common/hw-gfx-gma.ads b/common/hw-gfx-gma.ads
index 76424d6..c33c513 100644
--- a/common/hw-gfx-gma.ads
+++ b/common/hw-gfx-gma.ads
@@ -276,11 +276,10 @@
 
 private
 
-   PCI_Usable : Boolean with Part_Of => State;
    use type HW.PCI.Index;
    procedure PCI_Read16 (Value : out Word16; Offset : HW.PCI.Index)
    with
-      Pre => PCI_Usable and Offset mod 2 = 0;
+      Pre => Offset mod 2 = 0;
 
    ----------------------------------------------------------------------------